DESCRIPTIONS OF SPECIES. 435 



Pinna, and I will for the present accept Hyatt's assignment of it to the former genus. 

 The persistent differences in shape and surface between Aviculopinnaf peracuta and 

 the other Aviculopinnse would serve to discriminate two groups under Aviculopinna, 

 and they seem to me as constant and important as those upon which Sulcatopinna 

 was based. It should be remarked, however, that Pinna includes both shells having 

 an acute posterior cardinal angle, like Avlculojpinna peracuta, and an obtuse one, like 

 the other Amculopinnse. 



A very imperfect specimen oi Aviculopinna? percuiutah&s been obtained from 

 the Hermosa formation in the San Juan region. It has the smooth surface and 

 elongate hinge line, with forward sloping growth lines, which chai-acterize that 

 species in the Mississippi Valley. The convexity, which may have been reduced by 

 flattening, is in this specimen very slight, while the maximum height is 48 mm. 



Another still more fi'agmentary example has been obtained from the Weber 

 formation of the Leadville district. Its surface ornamentation is not retained, so 

 that an estimate can not be formed of its original shape. It appears to have been 

 a large, long form with subparallel upper and lower margins, and I doubt if it is 

 conspecific with the form from the San Juan. The shell, so far as preserved, is 

 composed largely, perhaps wholly, of a rather thick pi'ismatic layer. 



Locality and horizon. — San Juan region (station 220.5); upper portion of the 

 Hermosa formation. Leadville district (station 2257); base of the Weber formation. 



AVICULOPINNA NEBRASKENSIS Becde. 

 PI. IX, figs. 1, 2. 



1901. Aviculopinna nebrascemu. Beede, Kansas Acad. Sci., Trans., vol. 17, p. 186, pi. 13, fig. 1-ld. 

 Permian : Gage County, Nebr. 



The collections from Colorado occur in a dark, siliceous limestone of the Rico 

 formation, where the species is quite abundant. In size, some of the specimens must 

 have been at least 8 inches long — as large as the ordinary Pinna peracuta. The 

 growth lines are of course parallel to the lower and posterior mai'gins of the shell, 

 and are separated by intervals more or less proportional to its dimensions. They are 

 very crowded, somewhat indistinct, and occupy but a small .space along the infei'ior 

 margin, disengage themselves when thej'' assume an upward direction, and end 

 abruptly at the hinge line, which they meet at an angle of about fiO°. Where paral- 

 lel to the posterior margin, they are regularly spaced (about 1 mm. apart), and their 

 lamellose character is well shown. This appearance is present when the shell has a 

 diameter of about .3.5 mm. The shell substance is seen to be composed of two layers, 

 the outer one being distinctly prismatic, the prisms of course normal to the two 

 surfaces, and an inner one, which in these specimens is coarsely crystalline. The 

 surface of contact between these two structural elements is naturally one of cleavage, 



