No. 186.] 



83 



pearance of a false area on the ventral valve, somewhat similar to 

 Atrypa, the margin being thickened and grooved; but this does not 

 appear to be a characteristic or constant feature. 



The specimens of the interior, which have fallen under my ob- 

 servation, usually preserve only the short crural processes; and it 

 is in one specimen alone, which is partially filled with crystalline 

 matter, that distinct cavities can be seen corresponding with what 

 I have described. The crystalline matter was flrst deposited upon 

 these internal organs, which have subsequently almost entirely 

 decomposed, leaving in the cavities fragments of the substance, 

 showing the original form of the crura and appendages. 



The shells of this form have been described as Terebrattjla, and more 

 recently have been included with Rhtnchonella, from which they differ 

 conspicuously in the great inequality of the valves and the extension of the 

 hinge-line, as well as in their internal structure. 



The accompa,nying illustrations show all that has been thus far determined 

 relative to the internal structure 0/ this genus. Figures 1 , 2 and 4 are from 

 specimens of L. JiahelUtes ; and fig. 3 is a nearly perfect specimen of L. 

 Jimhriata. 



I I 



Tig. 1. The interior of the ventral valve, showing the cardinal teeth and the muscular 

 impression. 



Fig. 2. Interior of the dorsal valve, showing the cardinal process j, the teeth sockets 

 b b, the bases of tlie crura c c, and the short thickened septum which 

 divides the muscular impression. 



Fig. 3. An enlarged figure of L. fimbriata, showing the usual form, the foramen at the 

 beak, and the fimbriie on the cardinal margin. 



Fig. 4. The interior as shown in a specimen of L. flabellites, where it has been partial- 

 ly filled with crj'stalline quartz. Tiie decomposition of the crura, loop, and 

 tlie flattened disc, have left cavities of the same form in the quartz, and 

 small portions of the substance of some of these parts still remain. The 

 specimen is a dorsal valve which retains the matter formerly filling the shell, 

 aud it is seen from the ventral side. 



I liave heretofore referred to this genus the Leptoccelia {Atrypa ) dis- 

 parilis of the Niagara group, the Terebratula lepida of Goldfuss, the 

 T, sublepida and T. duhoisii of MM. Murchison, de Vernetjil and 

 Keyserling (Geology of Russia and the Ural Mountains). The L. concava 

 and L. imbricata of the Lower Helderberg group are analogous forms, 

 whiclr have been referred to this genus. The Atrypa hemispherica of 

 MiJRCHisoN is apparently a characteristic form of this genu?, as well as 



[Assembly No. 186.] 5 



