CYRENA. 23 



This species varies very much in external appearance, being, at 

 times, more convex than the specimen from which this description 

 was prepared ; I have a young G. mexicana wliicli is quite glo- 

 bose. The epidermis is usually light yellowish-green and smooth, 

 but in some cases it is of a dingy gray color and rough. 



The original type of Mr. Sowerby's G. mexicana having been 

 lost, some confusion has arisen as to the identification of this spe- 

 cies ; the description he gives of it, though quite short, contains 

 sufficient, taking the locality into consideration, to induce me to 

 believe that the shell under consideration is the one he had in view 

 in describing his species. 



Mr. Carpenter includes under the head of G. mexicana the G. 

 altiUs, Gould, or rather the G. tn'angula, v. d. Busch, as it 

 should be called, both being identical. I have examined an ori- 

 ginal specimen of G. altilis, and am convinced that it is diffe- 

 rent from G. mexicana. I have also examined, at the State 

 Collection in Albany, a complete suite of G. mexicana arranged 

 by Mr. Carpenter himself, and find that though at times certain 

 extreme forms of G. mexicana and G. triangula bear great 

 affinities to each other, the two types are very distinct and may 

 be separated without much trouble. The G. triangula is always 

 more triangular, larger and more solid than G. mexicana, and 

 its beaks are more prominent. 



The G. jloridana, which Mr. Carpenter places under the syno- 

 nymy of this species, is an entirely different shell. 



I have seen original specimens of G. fragilis, Desh., and of 

 G. sequ Hater alis, Desh., in the Cuming Collection, and find them 

 identical with G. mexicana. The G. sequilateralis is assigned 

 as coming from Guyana, an error, in my opinion. The G. mexi- 

 cana is, on the whole, an attractive species ; it is found quite 

 abundantly. 



19. Cyreiia californica, Pkime. — Shell ovate-subquadrangu- 

 lar. transverse, ineqiiilateral, tumid, somevrhat heavy ; anterior side pro- 

 duced towards the upper part, obliquely subtruncate, posterior side broadly 

 truncated towards the upper part and angular towards the inferior part, 

 basal margin rounded ; beaks not much raised, somewhat oblique, eroded ; 

 strife light, irregular ; epidermis yellowish-green ; valves white inside with 

 violet on the margins ; hinge-margin somewhat broad ; cardinal teeth diver- 

 gent, narrow, approximate at base ; lateral teeth unequal : anterior tooth 

 thick, conical, short ; posterior tooth narrow, placed at a greater distance 

 from the cardinal teeth. 



