Jurassic Mollusca from Arabia. 15 



By careful development it has been possible to almost com- 

 pletely expose the lateral area and a portion of the periphery 

 (PL III. tigs. 1 a, b). In its crushed state the fossil has the 

 following dimensions: — diameter of outer whorl 92 mm. (1) * ; 

 height of outer whorl 30 (0*326) ; width of umbilicus about 

 39 (0*42) ; the thickness of the whorl being indeterminable. 

 There are 42 or 43 principal ribs in the outer whorl ; each, 

 after traversing the narrow umbilical wall of the whorl, 

 passes forward in crossing the lateral area and bifurcates at 

 about the middle of this area into two equally strong ribs, 

 which cross the periphery without any interruption in an 

 orad-convex curve. Occasionally, but very rarely, there is a 

 principal rib which does not bifurcate. There is no trace of 

 the suture-line. 



The fossil is most probably referable to J. de C. Sowerby's 

 Ammonites torquatus. The type-specimen, which came from 

 the " Desert N.E. of Cutch " t, has the following measure- 

 ments : — diameter of shell 64'5 mm. J (1); height of outer 

 whorl 21*5 (0-33); thickness of outer whorl 23(0-35), and 

 width of umbilicus 28 (0'43). The outer whorl bears 41 

 principal ribs. Thus it will be seen that both in measure- 

 ments and ornaments the Arabian shell approaches very 

 closely the Cuteh form. In his work on the Jurassic Cepha- 

 lopoda of Cutch, Waagen§ figures only a large completely 

 septate example, and states that all the specimens in the 

 Museum of the Geological Survey of India came from " the 

 coarse red iron-sandstone of the Katrol range, that is, from 

 the middle region of the Katrol Group." It is stated to be 

 one of the commonest Ammonites of the Katrol Group ||. 

 The ornaments of the Arabian fossil agree very closely with 

 those of the larger of the two specimens (figs. 6, 6 a) from 

 the Spiti Valley figured by H. F. Blanford Tf, and referred by 

 him with a query to Sowerby's species. Unfortunately 

 neither the suture-line nor the form of the transverse section 

 is available for comparison in the Arabian fossil. The 



* The numbers in parentheses accompanying the dimensions measured 

 in millimetres indicate the proportion of each of them to the whole 

 diameter taken as unity. 



t Op. cit. p. 719. The specimen is now in the Museum of the Geolo- 

 gical Society of London. 



X The specimen is 68 mm. in diameter, but owing to the imperfection 

 of its anterior end its dimensions can be best taken at a diameter of 

 64 '5 mm. 



§ W. Waagen, op. cit. pt. 2, 1875, p. 191, pi. liv. 



|| R. D. Oldham, Manual Geol. India, 1893, p. 222. 



51 H. F. Blanford, Journ. Asiat. Soc. Bengal, vol. xxxii. 1863, p. 130, 

 pi. iii. figs. 6, 6 a, 7,7 a, 8. 



