48 rLATYCRFDINAv IIEXACRTXUS. 



Genus III. HEXACRINUS. (Austin.) 



Definition — Dorso-cential plate tripai-tite and hexagonal ; perisomic plates six, five 

 of which bear the rays ; mouth sub-central and unobtrusive. 



Mr. Phillips has admitted these very interesting fossils from the Devonian strata into 

 the genus Platycrinus, on the supposition that the dorso-central plate of that genus was 

 tripartite, an error which has been removed by the fortunate discovery of numerous 

 specimens in such a state of preservation as to preclude the possibility of further doubt 

 on the subject. 



The only point on which the two genera agree is that of possesing but a single series 

 of perisomic plates, and which bear the rays. In every other respect they are generically 

 distinct. 



Animals of the genus Hexaerinus though rather numerous during the accumulation of 

 the Devonian deposits, do not appear to have existed in the succeeding epoch, for as yet 

 not a single specimen of the Hexaerinus has been observed in the carboniferous limestone. 

 Nor has the genus been discovered in the Silurian deposits. 



1. Species. Hexacrinus melo. {Austin.) 

 PL VI. fg. \.a,h,c, d, e. 



Definition — General figure globose, but somewhat elongated towards the columnar 

 point of adherence. — Dorso-central plate tripartite, hexagonal and conical ; on each face 

 of the hexagon rests a perisomic plate. These six plates are tumid on each side the 

 orbicular surface ; the five which bear the rays are larger than the non ray bearing plate. 

 All the sutures are sulcated, and the plates studded with granulas though without much 

 symmetry as regards their arrangement, the granulae are generally most numerous on 

 the upper parts of the perisomic plates, and on the lower angles of the dorso-central 

 plates. The abdominal plates are large with a central prominence on each : one larger 

 than the rest crowns the summit. These plates are also granvilated. Mouth sub-central 

 and unobtrusive. Column and number of rays unascertained — the latter consist of five 

 groups as in the Platycrini. 



