INVEETEBEATE PALAEONTOLOGY. 199 



this genus, and presenting all of its external characters, it would seem to vary 

 considerably in the details of its hinge-teeth. He describes the anterior 

 cardinal tooth as being elongated and bifid, and says that sometimes the 

 posterior cardinal tooth is very elongated and linear in the right valve, and 

 in other examples nearly obsolete; while, he adds, that "in the left valve, the 

 posterior cardinal is always short and thick, sometimes grooved or bifid." 



In L. Carolinensis, Conrad, the only species of which I have had an 

 opportunity to see the hinge, the cardinal teeth of both valves are short, 

 ranging very obliquely forward and downward, quite close together, and are 

 very slightly diverging, the posterior one of the right valve being stronger 

 than the other, while both in the left are slender, the anterior one being less 

 prominent than the other. None of them show any indications of being 

 emarginate or bifid. Its lateral teeth are well developed in the left valve ; 

 the anterior one being more elongated, while the posterior is remote from the 

 cardinal teeth. These laterals fit into corresponding grooves* in the margin 

 of the other valve, above smaller laterals of the same. 



The genus Linearia is related to Arcopagia, under which most of the 

 European species have been described. It can be distinguished, however, 

 by its less orbicular form, generally without any traces of a posterior flexure, 

 and its usually radiately-striated surface, as well as by the differences in the 

 details of Fhe hinge-teeth. 



So far as known this genus seems to be confined to the Cretaceous 

 system. 



Line aria 1 formosa, M. & H. 



Plate 30, fig. 2. 



T, Vina formosa, Meek and Hayden (May, 1860), Proceed. Acad. Nat. Sci. Philad., XII, 179. 

 Abraf formosa, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 14. 



Shell transversely subelliptical, very thin, moderately convex; anterior 

 extremity very narrowly rounded ; posterior subangular ; dorsum sloping 

 gradually, with a slightly convex outline in front and behind the beaks; base 

 forming a regular semi-elliptical curve ; beaks small and almost exactly cen- 

 tral; surface apparently smooth, but under a good magnifier seen to be 



* These grooves in the margin of the right valve for the reception of the lateral teeth of the left 

 are represented much too small, and the anterior one much too short, in Mr. Conrad's figure of this 

 species in Professor Kerr's report. 



