70 



POTERIOCRINID.E. — POTERIOCRI.MS. 



Description of the Calcareous Skeleton. 



The Dorso-central Plate of the Poleriocrinus crassus is so much concealed 

 by the lower edges of the perisomic plates, and the upper columnar joint that it is 

 impossible to examine it in the best preserved specimen ; for unfortunately in tiiis case 

 the more perfect the specimen the less chance there is of ascertaining its structure, as 

 regards the maimer of its attachment to the column. 



The Perisomic Plates. — The first or lower series which surround the dorso-central 

 plate, and partly rest on the superior columnar joint consists of five pentagonal or irregu- 

 lar hexagonal plates. These plates bend under at an acute angle, and rest by a striated 

 border on to the superior columnar joint. 



The second series is also composed of five plates, but two of these are heptagonal, 

 and three hexagonal, with their lower prominent points inserted into the retiring angles 

 formed by the union of the first series; the third series or ray bearing plates consist also 

 of five, and as one of the second series is irregular in shape, a corresponding irregularity 

 occurs in one of the ray bearing plates to meet the anomalous modification. 



At one point of the circumference, and between two of the ray bearing plates, are two 

 or three irregularly shaped plates which cause a wider interval between the rays at the 

 point where they occur, than has been observed between the other rays. Various con- 

 jectures have arisen as to the use of these intermediate plates in the animal's economy. 

 As it is probable that the encrinites derived a portionof their food from the bottom, it is 

 obvious that if this open space was turned downwards the mouth, whether proboscidiform 

 or otherwise, would have a freer scope to search for objects of sustenance on the sea bottom 

 than if its base was completely surrounded with rays. This peculiar form may therefore 

 have been designed to enable the animal to obtain its food with greater facility. 



All the perisomic plates are granulated, either from the effect of weathering, or of the 

 original structure. 



The Oral Tube, or Proboscis. — One striking feature in the Poieriocrini, is the 

 absence of those plates which we have termed abdominal, and which Miller called 

 pectoral plates. 



Instead of the vertex being covered in by these plates, as in some other genera, the 

 proboscidiform oral tube in this, appears to occupy the entire space included within the 

 circle formed by the primary rays, and its immense size and length in comparison 



