256 The American Geologist. April, i89i 
relationships of this attractive, yet rather anomalous, group of echino- 
derms paleontology has aided in no small degree. In fact it is due chiefly 
to a study of the fossil forms that the present advanced state of the sub- 
jiit has been reached. And it may be truly said, in this connection, 
that no group of writers has done so much in this field as the American 
students of ancient life who have had occasion and good fortune to labor 
in the paleozoic horizons of the Mississippi basin. Among these ob- 
servers none have carried on such an extensive series of investigations 
and have obtained such marvelous results as Messrs. Charles Wachsmuth 
and Frank Springer, who have lately made another most valuable 
addition to a knowledge of the morphology of the crinoids. The results 
referred to are embodied in the memoir just issued, which may be well 
regarded as one of the most important contributions that has appeared 
since the days of J. S. Miller. 
The presentation treats chiefly of the structure of the ventral covering 
in the different groups of the crinoids ; of the plates between the rays ; 
and of the relations these hold to one another and to those of the tegu- 
ments. Hitherto it had been supposed by some authors that the disk, 
•or that part enclosing the visceral cavity, was, in many, if not all, of the 
paleozoic crinoids covered by a second integument, which was wanting 
in later and recent forms. As to the plates between the rays all writers 
have discriminated between "calyx" and "disk" inter-radials ; the 
former term being applied to the massive, well-formed ossicles of the 
paleozoic crinoids ; the latter to the small irregular perisomic pieces of 
later species. The calyx inter-radials, except those of the Apiocrinida?, 
were thought to be followed by a vault, the others by a disk. The 
studies of Messrs. Wachsmuth and Springer led them to suspect that 
these assumptions were not altogether correct; and a careful investiga- 
tion seemed to indicate that all plates lying between the i*ays and their 
subdivisions are parts of the same element; and that therefore the so- 
called "vault," as a distinct structure, has no place in crinoid morphol- 
ogy. It is also shown that, in paleozoic times, the rigid integument such 
as is found in the later Camarata gradually became evolved from the 
thinly plated disk of the earlier forms and that the heavy plates between 
the rays are exaggerated perisomic plates. By a comparison of the 
various Camarata it is found that during the Silurian the ventral surface 
in the majority of cases was composed of small irregularly arranged 
pieces; that these increased in size during the Devonian; and in the 
Carboniferous attained great prominence and rigidity, as is best shown 
by the Batocrinites and Actinocrinitcs. All stages of transition between 
the two extremes are readily traced. In regard to the ambulacra obser- 
vations show that these features may be tegminal or subtegminal, even 
among the forms of the same genus; and that they are more frequently 
exposed in the earlier crinoids. Among the latter the covering pieces 
are smaller, more regularly arranged and not so highly differentiated as 
in the later species. In this respect the resemblance to recent forms is 
very striking ; and except for the sutural union of the various plates 
and the closure of the mouth and food-grooves the conditions of the 
