392 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxi. 



apollini.s., are given on Plate XXVI, and of the type of JAngulcUa [L. 

 davisi) on Plate XX VII. Three other strongly marked forms of lAnf/u- 

 lella are illustrated on Plate XXVIII. 



The memoir of Mickwitz gives the genus Oholus a position that it 

 had not lield prior to this very thorough investigation. With his 

 descrii)tions and i)lates and a fine suit of specimens worked out from 

 material given me by J)r. F. Schmidt, I have been able to make a 

 series of comparisons that at times have led me to doubt the advisa- 

 bility of distinguishing Lingulella even as a subgenus of Oholus. This 

 distinction is based on the more elongate form of most of the species of 

 Lin(juleUa and the greater tl^i(;kness of the shell of the typical forms 

 of Oholus. There are differences in the position, size, and form of the 

 muscular scars, visceral area, and vascular canals of the two, but they 

 are not greater than those between diifereut species referred to Lingu- 

 lella or to Oholus. The same general arrangement of muscle scars pre- 

 vails, but on comparing the interior of the dorsal valve of 0. [L.) 

 davisi (Plate XXVII, fig. 4), with that of 0. {L.) aeutangulus (Plate 

 XXVIII, fig. 2), or O. (L.) ampins (Plate XX VIII, fig. 4), we tind as 

 great variation as when the comparison is made with the dorsal valve 

 of Oholus (Plate XXVI, fig. 2). The same is true of the ventral valve, 

 although the means of comparison are in this case not so good. The 

 oldest si)ecies of Lingulella {L. granvillvnsis) of the Upper Olenellus 

 zone has the outline of Oholus celatus, and the interior markings of the 

 ventral valve are also of the same type. 0. (L.) aeutangulus (Plate 

 XXVIII, fig. 1) has the heart-shaped pit so characteristic of Oholus 

 (Plate XXVI, figs. 1, 4, 5), and the arrangement of the muscular scars 

 is essentially as in Oholus, but the outline of the valve is much more 

 elongate. 0. (7v.) darisi and O. (L.) amplus vary decidedly from Oholus 

 in the interior markings, but not more than from O. (/>.) amplus and 

 0. [L.) aeutangulus. The variations are so well shown by the figures 

 on the plates that detailed descriptions will not be entered upon. 



The genus Hehmidtla Volborth is ccmsidered as a subgenus of Oholus 

 by Mickwitz. A comparison of typical specimens of ISchmidtia eelatus 

 and Lingulella davisi leads to the view that Schmidtia is identical with 

 Lingulella. 



