'222 PROCEEDINGS OP THE ACADEMY OF [1881. 



the variety thus produced has ])een described by Meek and 

 Worthen as B. trochiscus. It has a more spreading disk, a more 

 concave dome, a comparatively lower body, is of larger size, and 

 consequently has more interradials, but otherwise is not different 

 from B. Chrysti. B. planodiscus Hall, which occurs still higher 

 in the Burlington and Keokuk transition beds, and in the lower 

 part of the Keokuk Limestone, is evidently a more mature form 

 of B. Chrysti and B. trochiscus, which by enormous development 

 in the radial regions, and a great increase of interradial and inter- 

 axillary plates, attained a still greater expansion of the disk. In 

 B. Chrysti and B. iyouez, interaxillary plates are wanting; they 

 are occasionally represented b}' one or two plates in B. trochiscus, 

 while B. planodiscus has from nine to eleven, with a similar 

 increase of interradials. In the latter species, the small bifur- 

 cating arm pieces, from which in B. Chrysti the second arms are 

 given off, and also the two succeeding rows of pieces, in both 

 arms, are enclosed within the body walls, the inner row as radials, 

 the other as interradial or interaxillary pieces, which all attain 

 the form and size of the associated plates in the lower orders. 

 B. planodiscus has forty arms like B. Chrysti and B. trochiscus, 

 but they are simple, branching in the body; while the other two 

 species have twenty arms which branch in their free state. The 

 increase of arms no doubt takes place in this group in a similar 

 manner as in Actinocrinus and Strotocrinus, but while in the two 

 latter, the alternate pinnules of onl^- the two main divisions of 

 the ray became arms, in B. planodiscus the proximal pinnule of 

 each arm was thus transformed. 



In B. Chrysti and its allied forms, we find an illustration of the 

 difficulty we often encounter in discriminating between species 

 and varieties. There are apparently four forms represented in 

 that t_ype, of which the two extremes, viewed separately, are well 

 defined specifically as well as geologically, but placed in connection 

 with the two others, they form a series which might well be taken 

 for variations of one species. 



A similar case is presented by a series of specimens obtained 

 from the Keokuk Limestone of Indiana. The collection com- 

 prises nearly two hundred specimens of Batocrinus. but contains 

 comparatively few species. By far the greater number came from 

 Bono, Lawrence Count}^, others from Edwardsville, Floyd Count}', 

 a few from Canton, and the rest from Crawfordsville. The Bono 



