ANIMALS. 175 



looked, however, that both Families may have been much more closely connected by 

 means of palaeozoic forms, than they appear to be by such as are now living. The 

 moUusks of the various genera of Nuculida differ remarkably from each other in their 

 structure, perhaps more so than in any other known family ; siace in Leda, Yoldia, and 

 Solenella their mantle is closed posteriorly, and furnished with an ingress and an egress 

 siphon ; whereas in Nucula, the mantle is entirely open, and unprovided with these 

 appendages. These differences are indicated in the shells — for example, by the 

 presence of a sinuated pallial line in the former, and an entire pallial line in the latter. 

 Probably Leda and Nucula graduate into each other ; as a species shortly to be noticed, 

 strictly speaking, is neither of one genus nor the other. The shells of Nuculida, 

 although agreeing with those of Arcida in their dental system, differ in having an 

 arcuated hinge-line, and an internal cartilage confined to a more or less projecting 

 callosity. While Nuculidcs, as regards the cartilage-fulcra, is seemingly allied to certain 

 genera of the first section of De Blainville's Pylorides, Arcida is undoubtedly related to 

 the Monomyarians and certain Dimyarians, as M7/tilid(S nnd Bakevelliida, which have 

 the cartilage more or less expanded over the hmge-plates. 



The two following genera are all that are known as Permian. 



Genus Nucula, Lamarck. 



Diagnosis. — " Shell equivalve, inequilateral, shortened anteriorly, ovato-trigonal or 

 obliquely ovate, closed, smooth, or concentrically striated, or (in certain exotic and 

 fossil species) marked by zig-zag or radiating furrows ; always invested with a smooth 

 epidermis ; margin denticulated or smooth ; backs approximated, incurved ; inside 

 nacreous : hinge-line angulated, a ligamental fossette at the angle, and a range of 

 comb-like, small, sharp teeth on each side ; ligament chiefly internal ; pallial impression 

 entire."^ 



Nucula Tateiana,^ King. 



Diagnosis. — Wedge-shaped: smooth: very inequilateral : anterior margin shorter than 

 the posterior, and at right angles to the dorsal line : half an inch in width. TJmbones 

 moderately tumid. Hinge-line acutely angulated ; with five anterior and nine posterior 

 teeth. 



During my last visit to Humbleton Hill, and after my plates were engraved, I 

 was rewarded by the discovery of a single impression of the dorsal half of a true 

 Nucula. From a gutta-percha cast taken of this impression, I have been enabled to 

 draw up the above incomplete diagnosis, which I must leave to be more fully worked 

 out by others. 



1 Forbes and Hanley, British Mollusca, vol. ii, pp. 214-15. 



^ Named after my esteemed friend, Mr. George Tate, F.G.S., of Alnwick. 



