IN THE DEVELOPMENT OF COMATULA. 247 



he has seen them in the progress of development finally disappear by absorption. 

 Dr Carpenter's observations, which set out from a stage a little in advance of 

 that of our larva, will doubtless throw much light on the steps which inter- 

 vene between the stage here described and the fully-developed Comatula ; they 

 are illustrated by a beautiful series of drawings and preparations, and it is 

 greatly to be desired that their author may not delay long in giving them to the 

 public. 



One of the most interesting features in our little larva, is the light which it 

 seems capable of throwing upon the nature of certain extinct forms. The 

 description by Guettard,* in the middle of the last century, of a living speci- 

 men of Pentacrinus caput medusae, and the discovery by Thompson,! towards the 

 I beginning of the second quarter of the present century, that the ComoMlw, in 

 their young state, are stalked Crinoids like Pentacrinus, must be regarded as the 

 two grand steps in our knowledge of the morphology of the Ci'inoidea by which 

 we have been enabled to establish the relations which exist between the extinct 

 stalked forms and the dwellers in our present seas. 



It is however the normal types of the fossil Crinoids that our knowledge of 

 the living forms has hitherto chiefly tended to elucidate; and there are still 

 numerous aberrant types, such as Haplocrinus, Coccocrinus, Stephanocrinus, &c., 

 whose affinities can scarcely yet be regarded as in all respects satisfactorily deter- 

 mined. I believe that the little animal which forms the subject of the present 

 paper will help us towards a clearer conception of the relations of some of these, 

 and enable us to assign their true significance to certain points in their struc- 

 ture which have presented difficulties in the way of a satisfactory morphological 

 analysis. 



The Devonian genus Haplocrinus will here at once suggest itself— so remark- 

 able by its tall pyramidal roof and rudimentary arms. In this genus, the summit 

 segment of the stem undergoes no metamorphosis, and the centro-dorsal piece of 

 Comatula is accordingly wanting, while the stem is immediately surmounted by 

 the basilar zone, consisting of five basalia. To this succeed the five radii, rendered 

 irregular by two of these radii consisting each of a single radiale, while the three 

 others are each composed of two radialia. Lastly, alternating with the radii, are 

 the five sloping sides of the pyramidal roof, and these I think must be recognised 

 as the exact equivalent of the five valve-like plates of the pre-brachial Comatula, 

 and therefore inter-radialia, here separated from the basalia by the greater deve- 

 lopment of the radialia ; for not only their position, but the discovery by Johannes 

 MiJLLER of rudimental arms in this genus, is inconsistent with Romer's view, that 

 ' the roof-plates are the arms of Haplocrinus in a condition similar to that of the 



* Mem. de I'Acad. Roy. des Sciences, 1755. 



\ Memoir on the Pentacrinus europcBus, Cork, 1827; and afterwards in the " Edinburgh New- 

 Philosophical Journal," 1836. 



