204 Messrs. Austin on new Genera 



more equally developed than the intervening ones, and di- 

 vide the body into compartments. These series are easily 

 distinguished by their prominence from the intermediate 

 plates. A series of three traverse the lower portion of the 

 body ; these are succeeded by two other series of three each, 

 which branch off at a considerable angle and form the points 

 of attachment for the rays. Within the forks formed by these 

 branches are groups consisting of three or more smaller and 

 less prominent plates than those already described. In each 

 of the compartments between the ray- bearing series is a 

 group of several irregularly shaped hexagonal and pentagonal 

 plates, sometimes amounting to seventeen or eighteen; in other 

 cases the number is less, for greater irregularity is observable 

 in this genus than in any other yet described. Rays composed 

 of double series of joints. 



P. articulosus, Austin. 



The perisomic plates agree with the generic description, 

 which has, in fact, been taken from this the typical species. 

 Rays eighty, composed of double series of joints : the rays 

 may really exceed the number specified above, but we have 

 clearly seen their development up to that amount. Column 

 circular, and subject to the same periodical changes as in 

 other genera. 



P. costatus. 



The perisomic plates agree with the generic definition. 

 Mr. Phillips has in Murchison's e Silurian System' erroneously 

 described this species as Actinocrinites (moniliformis). To that 

 genus it bears no resemblance either in the number or ar- 

 rangement of the plates forming the calcareous framework. 

 To bring this species within the generic character of Actino- 

 crinites, it is necessary to consider the scapulae as situated at 

 least three series of plates below the true ray-bearing plates. 

 It is scarcely necessary to remark, that such a method of de- 

 termining species is quite unsatisfactory in every point of view, 

 and must lead to important errors if adhered to. 



Miller originally admitted it into the genus Actinocrinites, 

 on the evidence of a columnar fragment ; but as the form of 

 the column possesses no peculiarity of structure sufficiently 

 striking to warrant specific distinction, we have altogether re- 

 named it. 



P. globosus. 



The number and arrangement of the perisomic plates answer 

 to the generic type. Rays and column unknown. 



