10 LARVIFORM CRINOIDS 



dorsal cups are formed of a ring of low basals and five closely joined radials 

 with no sign of an anal structure; (2) the upper surfaces of the radials bear 

 well-developed articular processes and are extended horizontally into con- 

 spicuous muscle fields. But here similarities cease. The most conspicuous 

 difference is the absence of an oral crown in A. aitstinii. Carpenter and 

 Etheridge state that "there are no signs of its having been roofed over by a 

 closed dome or vault of any kind. Had such a structure existed within the 

 circle of radial plates, it would assuredly have been preserved in the original 

 of figure 2, together with the ring of first brachials." [2, p. 284.] If the 

 oral crown had formed as important a part in A. austinii as in our species it 

 would certainly have been preserved, as it is present in all of our specimens, 

 none of which possesses even the lowest brachials. Even if A. austinii passed 

 through a stage in its development when it possessed an oral crown similar 

 to that of some of the "immature" specimens, the fact that this oral crown 

 in the mature forms was no longer attached directly to the dorsal cup but 

 was separated from it by perisome is a distinction of generic importance 

 sufficient to separate it from species which at maturity retained the oral 

 crown firmly atached to the radials. A second conspicuous difference is the 

 unsymmetrical development of the radials in A. austinii, whereas in our 

 specimens pentamerous symmetry is nearly perfect. Even in A. jakovlcvi and 

 A. in flatus Wanner the axillary radials are only slightly wider above than 

 the radials bearing only one arm facet whereas below they are all of prac- 

 tically uniform width. A third point of difference is seen in the character 

 of the articular surfaces of the radials. Judging from Carpenter and Eth- 

 eridge's figures, A. austinii possesses a simple transverse ridge separating a 

 concave beveling of the outer edge of the radial, the dorsal ligamental fossa, 

 from the generally concave surface of the muscle field. The large opening 

 of the central canal occurs close to and often interrupts the transverse ridge. 

 This differs from our species in certain respects as may be seen by comparing 

 with the description of these structures presented on a subsequent page, as 

 well as by a comparison of figure Id on Plate II with figures lb and 6a of 

 Carpenter and Etheridge on their Plate XV. 



The question as to whether any of these recently discovered species 

 should be referred to the genus Allagecrinus has been considered by Wanner 

 \ 17, pp. 11-12] who concluded that there is not sufficient basis for separating 

 them. However, Wanner took into consideration only two characters, num- 

 ber of arms and relative breadth of articular facets, and he is probably cor- 

 rect in his judgment that neither of these characters is of sufficient importance 

 to warrant a division. Nevertheless there are several other characters to be 

 considered. It is not known that Allagecrinus austinii (restricted) possessed 

 an oral dome attached to the radials and until it is established that such a 

 structure was present and was similar to the dome surmounting the more 



