306 CRETACEOUS PELECTPODA 



4. Leila, Gray, 1842. Elongated, thin, inequilateral, with the beaks some- 

 what tumid; hinge line straight, edentulous; type, X. BlainvilUana, Lea, from 

 South America, wherefrom also two other species are quoted. 



The shells of these two last named genera are in form almost identical with 

 some of the species of Anodonta. 



5. Triquetra, Klein, 1753, fllyria, Lam., 1819). Shell sub-trigonal, in- 

 equilateral, the dorsal edge of the hinge-line being more or less straight and 

 prolonged into a wing on either side, and generally somewhat longer than the 

 shell itself, which is of rather a solid structure ; hinge-teeth elongated, similar 

 to those of Unio, but strongly crenated or grooved, two anterior and one posterior 

 in the left valve, the reverse being the case in the right valve ; surface smooth or 

 corrugated; type, T, subvij^idis, Klein, Hanley (in Heeve's Conch. Icon.) des- 

 cribes twelve species from South America. 



6. Arconaia, Conrad, 1865, (Am. Journ. Conch., i, p. 234). Shell very elon- 

 gated, somewhat contorted, solid ; dorsal edge of the hinge line straight or nearly 

 straight, shorter than the shell and slightly winged ; hinge with anterior robust, 

 short, and finely rugose teeth in each valve, posterior teeth elongated and minutely 

 rugosely striated. As the type of this genus Conrad quotes Triquetra lanceolata, 

 Lea, from China, being the same as T. contorta, Lea, described in Reeve's Conch. 

 Icon, under the genus Syria. The distinction from this last genus is only based 

 upon the somewhat different shape of the shell and the presence of two anterior 

 teeth in both valves ; these are very variable characters, and it yet remains to be 

 proved whether a generic importance can be assigned to them. 



7. Castalia, Lam., 1819. Shell ovoid or sub-trapezoid, solid, tumescent, with 

 incurved beaks, inequilateral ; hinge line curved ; hinge with two urCequal anterior 

 and one strong elongated posterior tooth in the left valve, the opposite being the 

 case in the right one, all are transversally striated ; generally there are also one or 

 two small tubercular teeth below the umbones, more or less distinctly separated 

 from the anterior teeth; type, G. amUgua, Lam. Hanley describes in Reeve's 

 Conch. Icon thirteen species, mostly from America. Moussonin Mai. Blaetter, xvi, 

 p. 185, notes an additional one, G. ecarinata. 



XXXII. Family,— JETELJBIIDJE. 



The animal of ^theria, constituting the type of this family, possesses a large, 

 thick, oblong body ; the labial palps are large, semioval, attached by the straight 

 sides ; the mantle-lobes are disunited ; the two gills on each side are partially at- 

 tached to the body and mantle, and united posteriorly. The foot appears to be small 

 in the young animal, but it becomes obsolete with advancing age. The shells when 

 young are free, of an elongately high ovate shape ; when adult, attached with one 

 valve and of an irregular form ; the hinge is edentulous and the ligament sub- 

 internal ; surface covered with an olivaceous epidermis, more or less foliaceous; 

 pearly within. There are no fossil species known belonging to this family. 



