platycrinid.e. rlatycuinus. 43 



Formation and Localities. 



Carboniferous limestone. — Bollancl ; and Meiidips. 



The lengthened tubercles on this species mostly range with considerable regularity 

 along the plates, chiefly radiating in lines from the columnar point of adiierence, to the 

 outer margin of tlie plate ; and from the base of the rays to the lateral edges of the 

 perisomic plates. Tlie calcareous strings which cross the sutures are unique and 

 characteristic of the laciniatus ; the same peculiarity of structure never having been 

 observed in others of the genus. These calcareous ligatures serve to connect the plates 

 more firmly together, and they may be considered as so many prolongations of the fine 

 striae on the edges of the plates, observed m other Platycr'mi. 



The mouth appears to have been central, but from the imperfect specimens hitherto 

 obtamed it is impossible to decide on its true position. — Tlie rays, column and base of 

 attachment are unknown. 



The contour of this species approaches to that of the strlatus, which circumstance has 

 caused some doubt as to the propriety of retaining it as a separate species. These 

 objections we consider as not well founded, as each of the specimens present characters 

 quite sufficient to constitute specific difference. The fine striae on the plates of one 

 species are quite dissimilar to the tubercles on the other : and though the delicate lines 

 on the plates of the striatus are occasionally broken, they never assume the form of 

 distinct and prominent tubercles, like those on the tuber culalus. The manner in wliicii 

 the sutures unite, is also perfectly distinct in each species. 



Malformed Platycrinus. 

 PI. V.Jig. 3,fl, b. 



The figure 3, a, and b, PI. V. represents a curious specimen of Platycrinus, with the 

 doso-central plate so concealed by the disproportioned column, that it is difficult to 

 detei-mine whether the lower edges of the perisomic plates rest on the superior columnar 

 joint, or on the hidden dorso-central plate. 



The thickness of the column ^s compared with the size of the body in this specimen 

 is strikingly conspicious. Other occasional peculiarities of structure are met with in the 

 Platycrini, l)ut as they present characters more remarkable for the oddity of their 

 appearance, than for the instruction tliey afford, we abstain from dwelling further on 

 the subject. 



