Genus Heteraster A. d’Orbigny.

Heteraster musandamensis sp. nov. (PI. XLIV, figs. 4 a and 4 b.)

Dimensions in millimetres.-

	Holotype.	Paratypes.
Length................	41	37	32	24	40
Breadth...............	36	33	27	21	38
Height.................	19	18	14	10	18

Specific characters.-

Form oblong, aboral surface flat or slightly inflated; in the latter case the apex coincides with the apical disc; oral surface flat. Apical disc placed posteriorly, the ratio of its distance from the posterior margin to the total length being 1 : 3. Sulcus shallow; anterior paired ambulacra long and slightly sigmoid, posterior pair short; periproct marginal; peristome situated very near to the anterior margin.

Detailed description.-

The apical disc is square to slightly rectangular in shape, and consists of four genital plates and a large central madreporite, behind which the posterior genitals are in contact (fig. 9). The genital pores are usually very large, but in some specimens are distinctly smaller. In one specimen (E 18335), where the genital pores are remarkably minute, the madreporite is larger than usual, and extends to the right so as reduce the right anterior ocular plate to insignificant size, and passes backwards to separate the two posterior genital plates (fig. 10). The genital plates on this specimen carry small tubercles and granules similar to those of the adjacent ambulacral areas. Since this specimen seems identical with the nine other examples examined in all other respects. I regard it as an abnormal development and not as a distinct variety.

Figs. 9 & 10. Apical disc of Heteraster musandamensis, x 12. [Remark: definitively not from the screen display]

The unpaired ambulacral area lies in a shallow sulcus, which is continuous to the mouth. The pores show the typical Heteraster character, the outer pores of the pairs being sometimes elongated and sometimes short or even round. This arrangement is quite irregular, one short alternating with one, two, three, or four long. The inner pores of the pairs are round, or slightly elongate. The anterior paired ambulacra are long and slightly sigmoid. The poriferous zones are strikingly unequal. The bar of test between the elongate pores is ornamented by a row of small granules. Owing to the anterior position of the peristome these areas take a forward direction below the ambitus. The posterior ambulacra are short, and have characters similar to those of the anterior ambulacra, except that in them the inner series of pores is the stronger. The ambulacral areas carry small tubercles, which are surrounded by circles of small granules. The interambulacral plates carry a large number of small perforate tubercles, but these have mostly been removed by weathering. The periproct is marginal, circular, or slightly oval transversely. The peristome is situated very near to the anterior margin. It is transversely elliptical, with a slight labrum, and of about the same size as the periproct.

Type-material.-

Holotype (Brit. Mus. sp. No. E 18336) and nine paratypes (E 18328—35 & E 18337).

Remarks.-

H. musandamensis is closely related to the well-known Aptian zone-fossil H. oblongus A. d’Orbigny, but it differs in having the apical disc placed much farther posteriorly and the peristome farther anteriorly. For this reason, it might have been assumed that H. oblongus is ancestral to my species, but the latter has the greater geological age. Enallaster peroni Ficheur (43, p. 590 & pl. x), from the Lower Aptian of Algeria, has similar relative positions of apical disc and peristome but it differs in that its sulcus is much deeper, and the posterior ambulacra are more widely petaloid. Ficheur was clearly not confident of the necessity for maintaining a generic distinction between Heteraster and Enallaster.

Occurrence.-

Very common in a rubbly limestone in the upper beds of the Musandam Limestone, east of Khassab village, Elphinstone Inlet.

PLATE XLIV.

Fig. 4. Heteraster musandamensis sp. nov. a=holotype [Brit. Mus. (Nat. Hist.) E 18336] aboral surface; b=paratype [E 18337] oral surface. (See p. 642.)

LEES G.M. [1928b].- The geology and tectonics of Oman and of parts of south-eastern Arabia. Quarterly Journal of the Geological Society, vol. lxxxiv, part 4 / Nº 336, pp. 585-670, 11 pl. (xli-li).
From pages 642 to 643, and 669.
Reprinted by permission of Geological Society of London © 1928.