62 S. LOVEN, ON 1^(JU11TALESIA, A GENUS OF ECllINOIDEA. 



rays, and so on, with successive little plates in all the geiieran; »the correspondence 

 between the plates that prutect the eyes in the Starfish and tlie smaller perforated 

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

 of these and ))the angles between the rays in the Starfish. » Austin ') was of opinion 

 that, in Cidaris, the ambulacra — in wliirli he seems to have included the ))ocellar» 

 pieces — »terminate near the apex, which is composed of live plates, each of which 

 lias a central opening or ovarial aperture. These pieces united may be considered as 

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

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

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

 system in the young Starfish, arrived at the conclusion that its central plate is a solidi- 

 fied Iiomologue of the basal plates, and that the set of five plates in the angles corre- 

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

 the Crinoid. Beyrich ■*) considered the apical system, with regard merely to its position, 

 as tlic analogon of tlu' C'rinoidean calyx. Ikit 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 i-emote geological period tliat the classes of the Echinodei-nis 

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

 impoi'tant characteristics, tlic actual pi-esence of whicii still holds together their diversi- 

 lied forms. Whenever, therefore, we are called uj)on to compai'e the leading features 

 of one class to those of anothei", we do well to trace them l)ack, as near as we can, to 

 that common source, for, close as presunjal)ly 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 own way, some by slow degrees, others 

 rapidly, every time that a new braiichlct of tlu' group has been developed, and it has 

 become a delicate task to parallel features that perha[)s have I)eeu only slightly 

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

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



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

 of: a centi-al pentagonal ossicle; contiguous to each of its five sides one of five other, 

 hexagonal ossicles, fui-ming a closed ring; and, in the outer angle between every two 

 of these, one of a second, external, set of fi\c pentagonal ossicles. This is tlie general 

 formula, which in the Kehinoidea has remained, more or less altered, but ahvays re- 

 cognisable, from Paheozoic to recent time. If we look lor it in the Crinoidean calyx, 

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

 Meso/oic, but it is oidy when we approach the older rala-o/.oie time that forms come 

 in sight by which we are led to ex|iect to see it elearh e\|ii'essed in some early genus, 

 coeval in a certain deoi^n' with the oldest nf the Ivdiinoidea. it seemed to me that 



') Ann. Nat. Hist., 2:d Ser., VIIl, 285, 288; 18.5.5. 



-) Proo. Am. Ac. Arts and So., 1863, A pi-. 14. — Einliryolowy of St;ir(isli. Coiitrihntions to the Nat. 



Hist. United States, V, 1804, .50; lleprint 1877, G2. 

 •') Ueber die Basis dur Grinoidoa Brachiata, Monatsbericlito Akad. Wiss. iScrlin, Fcbr. 1871. 



