434 OARBONIFKROUS FORMATIONS AND FAUNAS OF COLORADO. 



strongly sugt^vstod in li is roniarlv that A'Diculopinna is "'ii true Aviculoid, both on 

 account of the anterior wini;' and also because, as stated bv Meek, his Aviculop/nna 

 umericana has the jji'isniatic layer which is characteristic of this group." Wiiat 

 Meek does say in the passage refcncil to is that '' it has the prismatic shell structur(! 

 of the ix\\\o& Pinna and Aricida groups." Agreeing with this expression, nearly 

 ail general treatises describe the shell of l)oth the Aviculidic and the Pinnida' as 

 prismatic. 



Between shells referred to Avicidopinna? peracutn and Avimdopinna some 

 well-marked and constant differences are maintained. Perhaps the most obvious of 

 these is the character of the surface ornamentation, Aiticulopmnaf per acuta being 

 nearly smooth, while the Aidailopinnas are marked by strong, equally distant, 

 lamellose projections. The direction of these, and the .shape of the posterior 

 extremity also seems to constitute a rather constant differentiating character, since 

 in Aviculopinna? pm^acuta the posterior cardinal angle is more or less acute, while 

 in Aviculopinna it is sometimes obtuse, giving the outline below a backward swing. 

 The apex in Aviculopinnaf peracuta has never, I believe, been observed, so that 

 it is not known whether tlie beaks were terminal or not. The convexity of the 

 valves probabljr does not foi"m a reliable character, since in these shells the 

 curvature is often modified b_y compression. 



It appears that the shell structure of Pinna and Aviculopinna are practically 

 identical, the relative extent of the nacreous and prismatic layers not differing 

 materially in the fossil and living forms. The nacreous layer in Aviculo2Jinna appears 

 to be entire, and this circumstance would ally the genus rather with Atrina. than with 

 Pinna itself. The accepted difference as to the position of the umbones in the two 

 genera, on the strength of which chiefly the genus Aviculojnnna was proposed, can not 

 now be proved, and is perhaps not real. On the other hand, the surface ornamentation 

 of Aviculopinna is not known in either Pinna or Atrina, and seems to be quite 

 characteristic of that group, and to a less degree the straight, inferior margin and 

 regular slowlj^ enlarging shape and the linear thickening along the cardinal border. 

 It seems to me, therefore, that the uniting of Pinna or Atrina and Aviculoj^imm is not 

 to be considered. A riculopiinnaf peracuta seems to agree with Atrina and Aviculo- 

 pinna in the undivided condition of its nacreous layer, while there is nothing' distinc- 

 tive in the relative extent of it compared with the prismatic layer. In the gradual 

 regular enlargement of the shell, and in possessing a cardinal thickening, this species 

 is allied to Aviculopinna and differs from Pinna and Atrina. The nearly smooth 

 surface and extended cardinal line distinguish it stronglj' from the Aviculopinna 

 group, but the unplicated, unstriated surface distinguishes it from the living Pinna 

 and Atrina, almost equalh^ with the lamellose striation of Aviculopinna. Avicttlo- 

 pinna? peracuta seems to be more closelj' allied to Avic^dopinna and Atrina than to 



