/ 



106 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



whole test, to the bases of the free arras, extremely rigid. The interbrachial 

 plates of the regular sides generally comraence with one plate, which in most 

 families rests upon the radials and between the costals. It is usually fol- 

 lowed by two in the second row, and two, three, or four, according to spe- 

 cies, in any succeeding ranges there may be. The posterior interradius is 

 frequently wider, and divided vertically into two halves by a continuous 

 or interrupted series of anal plates ; but it may be constructed like the other 

 four. In the Platycrinidae and Hexacrinidse the first row consists of three 

 pieces, which are for the most part neither entirely interbrachial nor entirely 

 interambulacral. In the Khodocrinidse the first plate interradially disposed 

 goes down to the basals, thus separating the rays to their full length. This 

 first plate is therefore not strictly interbrachial in position, as it lies below 

 the horizon of the lowest brachials ; nevertheless it unquestionably belongs 

 to the same system of supplementary plates, which in this group extend 

 down to a position between tlie radials. There is some variation in the 

 extent to which the radials are parted by these intervening plates. In some 

 species of the Ehodocrinidae, exceptionally, the radials are only separated to 

 half their length, the variation occurring among different specimens of the 

 same species, and even in different areas of the same specimen. We find it 

 convenient to call these plates interposed between the radials "first interra- 

 dials" where it will avoid circumlocution. The interbrachial and interaxillary 

 plates of the Eeteocrinidae, contrary to those of all other Camerata are ill- 

 formed, and irregularly arranged. 



The interradial plates occupying the ventral side of \h^ calyx are as rigid 

 as those of the dorsal side, but are as a rule less regularly arranged. They 

 either extend up to the orals, or, when these are not represented, and the 

 ambulacra are subtegminal, they cover the whole tegmen, leaving no opening 

 except the anus. 



At the inner floor of the tegmen, we find in most of the Actinocrinidge 

 and Batocrinidae shallow grooves or open galleries, which are well shown by 

 the natural casts figured on Plate IV. Figs. 1, 2, 4, 5, 6, and 7, in which they 

 are represented by the elevations. These galleries pass out from near the 

 centre to the arm bases, and lodge the ambulacra] tubes. Alongside of them, 

 and sometimes covering them, there is frequently in both families — but, so 

 far as observed, only in certain genera — what appears to be a second integu- 

 ment (Plate y. Figs. 13, 15, 16, and 17), lying parallel to the inner floor, 

 which was formerly supposed to represent the ventral disk. The outer 



