274 



ON A DIBRANCHIATE CEPHALOPOD, STYRACOTEUTHIS 

 ORIENTALIS, n.gen. & n.sp., FKOM THE EOCENE OF ARABIA. 



By G. C. Ceick, F.G.S. 



Mead lOth March, 1905. 



The specimen described in the present paper formed part of a series 

 of fossils/ consisting chiefly of the casts of Gastropoda, collected by 

 Lieut. -Colonel Dr. A. S. G. Jayakar from the Eocene beds at Ras 

 Ghissa and Sharkeeyab in Oman, Arabia, and presented by him to the 

 British Museum. This fossil was obtained at Sharkeeyab, and is 

 preserved in a fawn-coloured marly limestone. From the character 

 of the fossils with which it was associated there seems to be no doubt 

 as to its geological age. 



The specimen is belemnitiform, 74 mm. long, conical, gradually 

 tapering for about three-fourths of its length, the apical portion 

 tapering rather more quickly, and terminating in a somewhat obtuse 

 point. It is a little compressed, its ventro-dorsal and transverse 

 diameters, at the anterior end, being 17 and 15 mm. respectively. Its 

 transverse section is oval or subtriangular, the siphuncular or ventral 

 side - corresponding to the base of the triangle. At the anterior end 

 of the specimen the alveolar cavity is displayed, the guard being 

 thickest on the ventral side. One side of the anterior part of the 

 cavity is pushed inwards and broken (see Fig. E). On each side 

 of the median portion of the dorsal area, the guard, for two-thirds 

 of its length, is a little flattened, or even slightly concave. 

 Symmetrically placed on the ventral surface, and 9 mm. apart, are 

 two strong sharply-incised grooves, which pass backwards fi'om the 

 edge of the alveolus, where they are about 1 mm. wide, for a distance 

 of 55 mm. and 47 mm. respectively, when each gradually dies away. 

 In a ventral aspect of the guard, the one on the left, i.e. the longer 

 of the two, turns towards the middle of the ventral surface as it is 

 disappearing ; that on the right gives off, throughout the lower half 

 of its course, branches which also pass on to the ventral surface. 

 A transverse section of the guard, at about its mid-length (see Fig. F), 

 shows that a crack extends from the bottom of each groove to the 

 boundary of the phragmocone, the crack being filled with a material 

 of a dark reddish-brown colour. The internal portion of the 

 guard between these two cracks is traversed by a number of small 

 similarly coloured and radially disposed cracks, but these, excepting 

 perhaps the median one, only extend part of the way between the 



1 This collection of fossils is mentioned by Mr. R. B. Newton, Quart. Journ. Geol, 



Soc, vol. Ixi (1905), p. 158. For details of the geology, see H. J. Carter, 

 " Memoir on the Geology of the South-East Coast of Arabia " [reprinted with 

 alterations and additions from the Journ. Bombay R. Asiatic Soc, 1852, 

 vol. iv], Geological Papers on Western India, etc. (1857), pp. 551-627. 



2 As in Belemnites, the side on which the siphuncle is situated is here called ventral. 



