26 PLATYCRINIDiE. PLATYCKINITES. 



Description of the Calcareous Frame Work. 



The Dorso- central plate. — This jilate differs from the dorso-central plate in the 

 typical species iii being conical instead of saucer shaped, so that the point of adhesion 

 for the column is prominent in the elongatus, while it is more fi-equently depressed in 

 the Icevis. The central apertiu-e communicating with the columnar canal small and 

 circular. 



Perisomic plates. — The number of these plates vary in different sjjecimens. In 

 some instances the body is surrounded by five, and in others by six ; but in either case 

 they are much elongated in comparison with other Platycrinites, and but slightly 

 spreading towards their summits. Theii" form nearly approaches to that of lengthened 

 parallelograms ; and from tlieii- comparative length they present characters which easily 

 distinguish the species from others of the genus. The excavations for the attachment 

 of the rays are shallow, and do not extend to a third of the width of the plates. The 

 lower edges by which they fit into the dorso-central plate are slightly rounded, and the 

 whole ^dsceral cup is less spherical than in most other kno\\ii Plati/crmites. AH the 

 plates are perfectly smooth. 



Mr. Phillips, in his Geology of Yorkshire, mentions that this species has an " inter- 

 scapular plate attached to the pelvis." However correct this remark may be as regards 

 some specimens, the same peculiarity is not universal, for as many individuals are met 

 with wanting the additional plate, as those which are found to possess it. Whether 

 the presence or absence of the additional perisomic plate is of itself sufficient to consti- 

 tute a specific difference, is a question wliich may be fairly entertained ; but as we are 

 opposed to the system of multiplying species, as well as genera, we continue to consider 

 it more as an accidental variety than as a distmct species. In every other particular the 

 specimens agree ; the shape of all the jilates, with the articulations for the rays and 

 column presenting no distinctive differences on which to found a species. 



The Meso-plates. — Unknown ; but they were evidently small, as indicated by the 

 form and arrangement of the perisomic plates, and the small excavations for the rays. 



The Abdominal plates. — From the weathered and altered state of the abdominal 

 plates in the only specimen in which we have met with this portion of the animal, we 

 are unable to detail their structure; but the}-, in the aggregate, formed a cone, fi-om the 

 centre of which proceeds the proboscis or oral tube, as may be seen in the centre figure 

 of our second plate. The interesting specimen fi-om which our drawing was taken, is in 



