62 S. LOVEN, ON POURTALKSIA, A GENUS OF ECHINOIDEA. 



rays, and so oii, witli succev^^sive Jittle plates iii all the genera»; »the correspondence 

 between the plates that prutect the eyes in the Stariish and the smaller perforated 

 plates of the upper disk of the Echini», hs well as that between the »ovarial» plates 

 of these and »the angies between the rays in the Startish.» Austin *) was of opinion 

 that, in Cidaris, the arnbulacra — in whicli he seems to have included the »ocellar» 

 pieces — »tenuinate near the apex, which is eomposed of tive plates, each of which 

 has a central opening or ovarial aperture. These pieces united n:ay be eonsidered as 

 the dorso-central plate, in the centre of Avhich the vent is situated». He nowhere 

 inentions the five genital plates of the Echinoids as collectively representing the dorso- 

 central plate of Marsupites. Alexander Agassiz, ^) from his study of the »abactinal» 

 sj^stem in the young Startish, arrived at the conclusion that its central plate is a solidi- 

 fied homologue of the basal plates, and tha,t the set of five plates in the angies corre- 

 sponds to the interradial plates, and the arm-plates themselves to the radial plates of 

 the Crinoid. Beyrich ^) eonsidered the apical system, with regard merely to its position, 

 as the analogon of the Crinoidean calyx. But in no instance the comparison was 

 more than mentioned incidentally, well worthy as it seems to be of an examination 

 in detail. 



It was at a very reniote geological period that the classes of the Echinoderms 

 branched off from their ancestral trunk, at the same time inheriting in coramon certain 

 important characteristics, the actual presence of which still holds together their diversi- 

 fied forms. Whenever, therefore, we are called upon to compare the leading features 

 of one class to those of another, we do Avell to trace them back, as near as we can, to 

 that common source, for, close as presumably were, at that starting point of diverging 

 existence, their mutual resemblances, most of their members have ever since been 

 going on modifying themselves, each in its OAvn way, some by sIoav degrees, others 

 rapidly, every time that a neAV branchlet of the group has been developed, and it has 

 become a delicate task to parallel features that pei"haps have been only slightly 

 altered in some type of long-continued existence, with those deeply changed in another, 

 and that, may be, Avithin the course of a much shorter time. 



Typically the »apical» system of the Echinoidea is a radiate structure eomposed 

 of: a central pentagonal ossicle; contiguous to each of its fiA'e sides one of five other, 

 hexagonal ossicles, forming a closed ring; and, in the outer angle betAveen every tAvo 

 of these, one of a second, external, set of five pentagonal ossicles. This is the general 

 formula, Avhich in the Echinoidea has remained, more or less altered, but ahvays re- 

 cognisable, from Palaeozoic to recent time. If we look for it in the Crinoidean calyx, 

 Ave find it profoundly obscured in the Cenozoic forms, and discernible enough in the 

 Mesozoic, but it is only Avhen Ave approach the older Pala?ozoic time that forms come 

 in sight by Avhich Ave are led to expect to see it clearly expressed in some early genus, 

 coeval in a certain degree Avith the oldest of the Echinoidea. It seemed to me that 



1) Ann. Nat. Hist., 2:å Ser., VIII, 285, 288; 1855. 



^) Proc. Ara. Ac. Arts and Se, 1863, Apr. 14. — Embryology of Stariish. Contributions to the Nat. 



Hist. United States, V, 1864, 50; Eeprint 1877, 62. 

 •') Ueber dia Basis der Crinoidea Brachiata, Monatsberichte Akad. Wiss. Berlin, Febr. 1871. 



