MR. P. H. CABPENTEE OX THE GENUS ACTINOMETEA. 455 



in contact with the top stem-segment : in P. Miilleri they are in 

 contact for about half their length and then diverge, while in P. 

 Wyville-Thomsoni they are completely united with one another 

 along the whole length of their sides, so as entirely to cut off the 

 radial pentagon from the top stem-segment, just as in S. Jaegeri. 

 There can therefore be little doubt that the basals of Penta- 

 crinus are homologous with those of Solanocrinus, and therefore 

 analogous, as a whole, to the compound basals of Actmometra, 

 which are not entirely developed out of the embryonic basal plates. 

 Only their central part, the rosette, has the same origin as the 

 basals of Pentacrinus, with the inner or central ends of which it is 

 strictly homologous ; for the bifurcating nerve-cords proceeding 

 from the angles of the chambered organ have the same relation to 

 the rosette and basal bridge ofActinometra as to the united central 

 ends of the basals of Pentacrinus, which are perforated by bifur- 

 cating canals in which these cords are lodged. 



It would seem, in fact, as if in Pentacrinus and Solanocrinus the 

 embryonic basal plates became directly transformed into the basals 

 of the adult ; while in Comatula they undergo metamorphosis into 

 the central rosette by the absorption of the greater portion of 

 their dorsal or primary tissue, and the development of a secon- 

 dary ossification on the ventral side of the original plates. 



In Antedon rosacea the metamorphosis is much more complete 

 than in A. Escliriclitii and in Actinometra, in which last new ske- 

 letal elements are developed by a more or less complete tertiary 

 ^ossification in masses of connective tissue, that correspond pre- 

 cisely in position and also, to a certain extent, in shape with the 

 basals of Solanocrinus and Pentacrinus. These, being most pro- 

 bably direct products of the growth of the embryonic basals, are 

 therefore strictly homologous only to the rosette oi Actinometra, 

 although analogous in position to the whole circlet of compound 

 basals in this genus, namely to the rosette and basal star taken 

 together. 



The recent genus Comaster, or Goonatula multiradiata of Grold- 

 fuss*, from the Indian Ocean, has been considered by most authors 

 generically identical with the fossil Solanocrinus, on account of 

 the appearance of the basals upon the exterior of the calyx. 



The condition of the central ends of the basals, however, and, 

 in fact, of the whole calyx, is very remarkable and very unlike that 

 presented by any other Comatula with which we are acquainted, 

 * Petrefacta Grermaniffi, i. p. 202. 



