ANIMALS. 175 
looked, however, that both families may have been much more closely connected by 
means of palzozoic forms, than they appear to be by such as are now living. The 
mollusks of the various genera of MVuculide differ remarkably from each other in their 
structure, perhaps more so than in any other known family ; since in Leda, Yoldia, and 
Solenclla their mantle is closed posteriorly, and furnished with an ingress and an egress 
siphon; whereas in /Vucu/a, 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 lme 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 Nuculide, 
although agreeing with those of Arcide in their dental system, differ in having an 
arcuated hinge-line, and an internal cartilage confined to a more or less projecting 
callosity. While Wuculde, as regards the cartilage-fulcra, is seemingly allied to certain 
genera of the first section of De Blainville’s Pyloridés, Arcide is undoubtedly related to 
the Monomyarians and certain Dimyarians, as Wytihde and Bakevellude, which have 
the cartilage more or less expanded over the hinge-plates. 
The two following genera are all that are known as Permian. 
Genus /Vucula, Lamarck. 
Diagnosis.—“ Shell equivalve, equilateral, 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.”? 
Nucura Taterana, King. 
Divagnosis.—Wedge-shaped: smooth: very inequilateral: anterior margin shorter than 
the posterior, and at right angles to the dorsal line: half an inch in width. Umdones 
moderately tumid. inge-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. ¥Yrom 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. 
