1885.] NATURAL SCIENCES OF PHILADELPHIA. 27 T 



of the orals. Similar modifications occur in the number of 

 underbasals, and among them Carpenter admits three and four 

 plates ; but when we find the underbasals in Stemmatocrinus 

 evidently fused together to a single piece, he regards this as a 

 stem joint. 



Even the joints of the column are sometimes tri- or quinque- 

 partite, from the top of the column to the end of the rootlets, 

 and principally in Lower Silurian genera ; nevertheless the stem 

 joints of the Pentacrinoid larva, and those of the Neocrinoidea 

 generally, are undivided throughout. Are we to consider the 

 former as different elements from the latter because they are 

 composed of three or five pieces ? Or are we to regard the five 

 plates collectively as the homologues of the undivided joints of 

 recent and other Crinoids ? In the latter case, why should not 

 the dorsocentral, i. e., the terminal plate of the column, be divided 

 in one or the other species ? That the plate is undivided in the 

 Pentacrinoid larva, and in the few species of Pentacrinus in 

 which it has been observed, is by no means a proof that it is so 

 in all Crinoids. 



It has been stated by Carpenter that " the basals are within 

 the ring of radials, and next to the dorsocentral." This is no 

 doubt frequently the case, but is not the universal rule. In 

 the Rhodocrinidae and Reteocrinidae the interradials are placed 

 between the radials, forming with them a ring of ten plates 

 around the basals, while in the Acrocrinidse the radials are totally 

 isolated from the basals by a wide belt of plates, which, although 

 not true interradials, may be fairby compared with them (PI. 8, 

 fig. 1). 



At the oral side, the arrangement is fundamentally the same 

 as in the cahyx, as can be observed in species in which all summit 

 plates are fully developed. Frequently, however, the first and 

 second radials are orally unrepresented, when the third radials 

 occupy the same position as their representatives in the calyx, 

 which is the same as that occupied by the third summit radials 

 of Strotocrinus, etc. 



If the orals were represented by the proximals, the latter 

 should be succeeded in all cases by the radials, and not be 

 included in the same ring. There is not a single instance of 

 Crinoids known to us where either a radial or an anal plate 

 entered the basal ring, or where an anal plate entered the 



