192 p. HERBERT CARPENTER. 



cover the buccal membrane of an ordinary Urchin. But in 

 the remarkable form Leskia (Pcdaostoma) niirabilis, there 

 are only five plates on the buccal membrane. These are 

 large, triangular, and interradial in position, as they alter- 

 nate with the bases of the ambulacra. Here I believe we have 

 the key to the problem, one which both Gray and Loven 

 have attempted to use, and in two different Avays, neither of 

 which seems correct when viewed by the light of our present 

 knowledge. In 1851, Gray^ wrote as follows, respecting 

 Leskia : " In the form of the mouth and vent it has con- 

 siderable affinity with the fossil Cystidea of Von Buch, as 

 especially the genus Echinosphcerites." Some years later, 

 when Billings and others had attempted to show that the 

 so-called " ovarian pyramid" was really the mouth or mouth- 

 anus, Loven- compared its five valves to those surrounding 

 the mouth of Leskia, a point which seemed to give con- 

 siderable support to Billings' views. On the whole, however, 

 it seems most probable that Agassiz, Liitken, and Wyville 

 Thomson, are right in regarding the ovarian pyramid as anal 

 in function. Agassiz^ compares its five valves to the five 

 plates which cover the anal opening in many young Echini, 

 during a considerable period of their growth, but which ulti- 

 mately undergo much modification. 



Leaving the Cystids for the present and returning to the 

 simpler and more comprehensible recent Crinoids, I think 

 there can be little doubt as to the homology of the oral plates 

 of Hyocrinus and of the Pentacrinoid larva of Antedon, Avith 

 the similar and similarly situated plates in the actinostome 

 of Leskia. We have seen that the Crinoid skeleton may be 

 regarded as composed of three distinct systems of plates, the 

 apical, the oral, and the intermediate. The latter is 

 developed in an equatorial zone, occupying the larger or 

 smaller area of perisome which gradually appears between 

 the oral and apical systems of the larva. A general 

 homology (irrespective of details) between the apical systems 

 of Crinoids and Echini is now universally admitted ; and if, 

 as I have endeavoured to show, the five oral valves on the 

 actinal membrane of Leskia are homologous with the oral 

 circlet of a Crinoid, then the coronal plates of the Urchin 

 must represent those developed in the equatorial zone of the 

 Crinoid. A still closer resemblance in matters of detail 

 will be pointed out further on. 



In some young spatangoids {Brissopsis) the actinostome is 



^ Loc. cit., p. 63. 



- " Om Leskia mirabilis," loc. cit., pp. 43G — 440. 



^ Note on Loven's article on Leskia mirabilis, loc. cit., p. 243. 



