308 Proceedings of the Royal Physical Society. 



with the plane of the exterior of the valve, continued for- 

 wards on the interior surface of the valve for nearly two- 

 thirds of the distance between the boss and the front margin. 

 Immediately in front of the base of the boss is a depression 

 in which are placed the adductor muscles ; the oblique dental 

 sockets on each side of the boss are deep, large, and a little 

 triangular. The reniform impressions are a marked feature, 

 being very large and strongly auriform, and occupying the 

 greater part of the interior of the valve from close under the 

 hinge line to the front. They are bounded on their external 

 margins by grooves between them and the bevelled edges. 

 The latter bear granules and small elevations, indicating the 

 vascular system. 



Ols. — The characters of this very remarkable shell are 

 wholly and entirely different from the Devonian S. prochtc- 

 toides, which, according to Professor De Koninck, has been 

 found in Australia. In the first place S. Jukesii is an 

 exceedingly convex and gibbous shell, with a flat, or nearly 

 flat, dorsal valve, without any trace of the concavo-convex 

 outline of Murchison's species. The latter, speaking gene- 

 rally, assumes a much more transversely elongated appear- 

 ance than S. Jukesii. 



The alliance with the Permian forms S. Goldfussi and S. 

 lamellosa is closer, both in the form of the shell, and relative 

 convexity of the valves. Were we dealing only with the 

 external characters I should have felt much inclined to refer, 

 provisionally at least, the Tasmanian and New South Wales 

 shells to one or other of the species just mentioned. The 

 Australian form has the shape, rounded angles, occasional 

 indented front margin, short hinge line, convex ventral valve 

 with its shallow sinus, concentric lamellae of growth, and 

 characteristic spines of one or other of the above species. 

 When, however, we compare the internal characters of the 

 respective shells many points of divergence may be noticed. 

 Thus, in the great disproportion of the reniform impressions 

 appears to lie a well-marked difference. Their larger size, 

 different shape, and much greater development in aS'. Jukesii 

 will be at once apparent if the figures now given be compared 

 with those of S. Goldfussi and S. lamellosa in the monographs 



