84 Trans. Acad. Sci. of St. Louis. 



costae also grow smaller in size posteriorly, the largest ones 

 being near the center of the basal margin. In addition to the 

 costae the shell is marked with fine concentric lines and with 

 a few coarser lines or indistinct wrinkles of growth. 



Remarks. White and Whitfield's description of Aviculo- 

 pecten nodocostatus (= Pterinopecten nodocostatus) was based 

 upon two type specimens which are now preserved in the 

 paleontologic collection of the University of Michigan. One 

 of these specimens is a left valve from the Chonopectus sand- 

 stone, and the other is a right valve from the upper horizon 

 of the " Yellow sandstone." These two specimens appar- 

 ently belong to distinct species. The original description 

 agrees in the main with the latter one of the two specimens 

 and it will be retained as the type of the species. The speci- 

 men from the Chonopectus bed differs from P. nodocostatus 

 in its greater proportionate height, in having nearly twice the 

 number of radiating costae, and in lacking the conspicuous 

 nodosity of the costae from which the specific name nodocos- 

 tatus was taken. There are other differences in convexity, etc., 

 due to the fact that the two specimens are opposite valves. 



Except in size the specimen from the Chonopectus bed 

 agrees quite closely with the illustration of P. laetus Hall * 

 but is proportionately a little shorter. In general form it 

 agrees even more closely with P. dignatus Hall,t but not so 

 closely in its surface markings. Each of these New York 

 species, however, which may in reality be but one, is only 

 about one-half the size of the Burlington specimen. The 

 difference in size, however, can scarcely be taken as a good 

 specific character, and the Burlington specimen is provision- 

 ally referred to P. laetus. This species was first described 

 from the Marcellus shale of the Hamilton series in New York. 

 At a later date a shell from the Waverly series of Ohio was 

 identified with it by Herrick, but in all respects except size, 

 the Burlington shell agrees more closely with the New York 

 illustration, than does the Ohio specimen illustrated by Her- 

 rick. 



* Pal. N. Y. 5 1 . pl.l.f. 13. 

 t Loc. cit. pi. l.f. 14. 



