438 ME. F. A. BATHEK ON PETAL0CKINT7S. [Aug. 1 898, 



opening through the tegmen in others. It may also be noticed that 

 the anal plate in the dorsal cup of Arachnocrinus is usually smaller 

 and less regular than in Cyathocrinus, as shown in text-fig. 15. 



As concerns the arm-structure, I pointed out five years ago, with 

 no thought of PetalocHnus in my mind, that in Gissocrinus typus 

 and G. campanula the proximal brachials of each arm were disposed 

 very regularly, lay close against one another laterally, had flattened 

 sides, and appeared (perhaps were) laterally united by suture 

 (' Crinoidea of Gotland : I,' pp. 157, 162). Text-fig. 15, reproduced 

 from J. Hall, shows a feature that has not hitherto been noticed : 

 if the drawing be correct, then in this species, A^^achnocrinus bul- 

 hosus, the axillaries are compound plates ; thus the apparent lax 

 consist of the true lax and the right and left IIEr^ ; this follows 

 not only from the disproportionate length of the axillaries, but from 

 the notch in their upper angles, representing the original division 

 between the right and left brachials. 



Gissocrinus, including Arachnocrinus, is then a genus that tends 

 in the direction of Petalocrinus, as regards the structure of both 

 dorsal cup and arms. If net the actual ancestor, it at least suggests 

 the manner of evolution. 



Fetalocrinus has been compared by Mr. Weller with Orotalocrinus, 

 on the ground of its arm- structure, and with Platycrinus from the 

 appearance of the dorsal cup. The latter comparison is untenable 

 if the base be dicyclic ; in any case the evolution of arm-structure 

 in Platycrinus and its allies was in the direction of biserial, pinnu- 

 late, slightly branched arms, right away from the structure of 

 Petalocrinus. The arms of Crotalocr^inus resemble those of Petalo- 

 crinus in so far as the branches are laterally united. The union, 

 however, is by loose suture, is restricted to the distal ends of each 

 ossicle, and is combined with great flexibility of the whole arm-net 

 thus produced. The relation of the arm-net to the cup is also dif- 

 ferent from that of the arm-fan : by the partial atrophy or vertical 

 compression of many proximal brachials, IIBr, and even IIIBr or 

 lYBr, come to rest on the upper surface of the radial, while their 

 corresponding ambulacrals are incorporated in the tegmen. The 

 stages of this development are seen in Enallocrinus, as pointed out 

 by Wachsmuth & Springer.^ But from those writers I have always 

 differed, in that I have believed this to indicate the evolution of the 

 Crotalocrinidse from the Cyathocrinidse. Into this discussion, for 

 which I have long been collecting materials, it is impossible now 

 to enter. It is enough to have shown that the Petalocrinidae and 

 Crotalocrinidae are related only ift so far as they proceed from one 

 starting-point along asymptotic paths. 



The family Petalocrinidae, hitherto undefined, may 

 be diagnosed thus : — Cyathocrinoidea in which IBB are 

 minute and probably fused, and in which the branches of each 

 arm, from lax to the finials inclusive, are fused into a rigid 

 arm-fan, articulating with the cup by means of a free IBr^. 



1 ' Orotalocrinus : its Structure & Zoological Position,' Proc. Acad. Nat. 

 Sci. Philad. 1888, pp. 364-390, pis. xix & xx. 



