STRATIGRAPIIICAL DISTRIBUTION. 



31 



Sub-genus — Bdellacoma, Salter. 



Bdellacoma vermiformis, Salt. Arms long, bordered by short spines ; ambulacral 

 avenues wide and flat, with large alternating apertures for the suckers. It is doubtful if 

 the avenues are bordered by more than a single row of plates ; but as there is a double 

 set of tufts of spines, this is probable. The author observes, " the main character of the 

 species, however, and that which distinguishes the sub-genus, is the possession of scattered 

 clavate tubercles over the upper surface. These are nearly as long as the spines. The 

 ambulacral avenues too, appear to differ materially from those of Palaocoma, in which 

 they are remarkably narrow, and the plates close, while in Bdellacoma they are broad, and 

 the ossicles remote." 



Locality/. — Leintwardine, in Lower Ludlow Rock. 



Sub-genus — Rhopalocoma, Salter. 



Hhopalocoma pyrotecJmica, Salt. The sub-generic and specific characters of this fine 

 species, the author states, must be taken together, and reside in the distribution of short, 

 broad, clavate, and compressed spines over the upper surface and margin, rather more than 

 their own breadth apart, and set on at the intersection of the reticular meshes, which 

 cover the arms, and the angles between the arms, but which are quite absent from the 

 central portion of the disc. This central portion above, which corresponds to the wide 

 aperture of the mouth on the under surface, is covered only by scattered, stellate, calcareous 

 spiculse of large size. A closer reticulation is found on the portions between the arms, 

 and the meshes become square in a double row, down the middle of the rays, and appear 

 to correspond nearly in position to the ambulacral bones of the under surface. The latter 

 are very slender and remote, even more so than in B. vermiformis, and form a broad am- 

 bulacrum, with only a few reticular plates bordering it, which bear clavate spines at 

 intervals. The mouth angles project a good deal inwards, and are armed with short 

 combs of spines." 



Locality/. — Leintwardine, in Lower Ludlow Rock. 



Protastee, Forbes. 



T'he genus Protaster was instituted by Professor Forbes, to comprehend those Silurian 

 star-fishes which have a small circular disc covered with squamiform plates. The arms are 



