45i MB. p. H. CATlPEN"T!i;E ON THK GENUS ACTINOMETRA. 



centrodorsal piece with tlie pentagonal base of the calyx. In 

 one specimen of A. poli/morpha the author has found them to be 

 almost entirely" absent, while in others they are very large and 

 stout. Loven has found something of the same kind in Antedon 

 Eschrichtii ; but in both the specimens of this species which the 

 author has been able to examine, they were scarcely developed 

 at all. 



These tertiary elements in the basals of Actinometra are of ex- 

 treme interest ; for they are precisely similar in shape and position 

 to the basals of two species of the fossil genus Solanocrinus, viz. 

 8. costatus and S. scrohiculatus. 



In both these species there seems to be no rosette ; but the 

 basals consist of five prismatic rods in contact by their central 

 ends, and occupying five deep grooves on the ventral surface of 

 the centrodorsal piece. They extend beyond its margin, however, 

 and so become visible on the exterior of the calyx, which is not the 

 case with the rays of the basal star of Actinometra. Hence this 

 would seem to preclude the possibility of their being formed like 

 the latter, by ossification in the connective tissue of the synostosis 

 between the radial pentagon and the subjacent centrodorsal 

 piece ; so that they are probably the remains of the original em- 

 bryonic basal plates, which are represented in Actinometra only 

 by the rosette : and the rays of the basal star of Actinometra would 

 therefore not be strictly homologous with the rod-like basals of 

 Solanocrinus costatus, although the analogy in their position is 

 complete. 



The calyx of S. Jaegeri presents a great advance upon that of 

 8. costatus with respect to the development of the basals, which 

 ledPictet* to propose the erection of this species into a separate 

 genus. Instead of being long and narrow, and in contact only 

 by their central ends, as in 8. costatus, they are broad and 

 wedge-shaped, and in contact along their whole sides, so as to 

 form a complete calcareous disk entirely separating the radial pen- 

 tagon from the centrodorsal piece. 



This is precisely their position in Pentacrinus, though there are 

 but few species of that genus in which the basals are relatively so 

 large and complete as in 8. Jaegeri. In P. asteria and in the two 

 fossil species P. hriareus and P. suhangularis they are small and 

 cuneiform, and only in contact by their central ends, just as in 

 8. costatus ; so that the greater portion of the radial pentagon is 

 * ' Traite de Paleontologie,' torn. iv. p. 288. 



