Review of Recent Geological Literature. 257 
plates in the two groups would be nearly identical, at least in a large 
number of cases. From a comparison of young and adult individuals it 
is further assumed that in the former the ambulacra are often exposed 
near the margin of the tegumen while in the latter they are entire- 
ly subtegminal. Thus the tegminal ossicles encroach from each side 
of the ambulacra and finally close above them, crowding the ambu- 
lacral skeleton inward. The so-called radial dome plates which were 
always regarded as true vault structures are shown to be merely greatly 
modified covering pieces. 
Other crinoidal groups are also taken up. In the Fistulata no vault 
is recognized, but all the plates between the ambulacra and between the 
radials are considered as perisomic. In most species the ventral sac is 
porous, while in others there is in front of the inflation a perforated 
plate like the madreporite of the urchins. The Larviformia which are 
now restricted to forms with the ventral surface made up of orals only, 
without any perisomic pieces. From these observations the authors 
conclude "that there is everywhere but one integument covering the 
body; that the ventral pavement, although undergoing various modifi- 
cations in geological times, is a disk" and that all crinoids, recent and 
fossil, are constructed substantially on the same plan. All plates lying 
between the rays and also between the ambulacra are therefore to be 
considered as perisomic pieces. 
The second part of Messrs. Wachsmuth and Springer's paper is de- 
voted to the anal plates of crinoids; and is to a large extent a reply to 
Mr. F. A. Bather's recent article, in the Annals and Magazine of Natural 
History for April, 1890, on the origin and development of the anal 
plates of the Fistulata. This author regarded the Fistulata as having 
two distinct anal plates, the so-called "radianal" or azygous piece and 
the "braehianal" or special anal plate. As to the former, the authors 
seem to be in accordance with the view that it is the lower section of the 
compound right posterior radial which in some groups performed anal 
functions. As to the second plate the two conceptions differ very essen- 
tially. Mr. Bather appears to think that the braehianal was primitively 
given off from an axillary, as a plate morphological by corresponding to 
an ordinary brachial, and that in the course of its geological developmenl 
it changed its position and passed down from above, between tin 1 radials. 
to the basals. This is explained by an increase in the width of the ven- 
tral tube in later forms, which caused a sinking of that organ into tin- 
dorsal cup; and by the shifting of the radianal to one side from its 
position beneath tin' radial. .Messrs. Wachsmuth and Springer under- 
take to prove that this theory is based upon a misinterpretation of the 
functions of certain plates. They show that the right posterior radial of 
Iocrlnux, Heterocrinus and Hybocrin/us is no more an axillary than the 
radial of an AcWnocrinus, which on its upper sloping lace supports an 
ordinary inter-radial. They assert that there is no sinking of the anal 
tube, nor a shifting of the radianal. hut that, in those forms in which 
a widening of the anal area has taken place, a new plate was Introduced 
10 
