129 



we shall find beneath the cluster formed by the latter, an abbreviated column 

 froiu which they proceed. 



On dissection, we find that the Comatulae have series of calcareous secre- 

 tions or ossicula exactly similar in their general plan and arrangement to the 

 Crinoidea, and especially to the genus Pentacrinus, intended like theirs to 

 sustain muscular action. 



PENTAGONAL PLATE. At the base of the subglobose body of the Comatulae, 

 exists a pentagonal unperforated plate, slightly convex externally, and concave 

 on the inside. It is analogous in situation to the first columnar joint of the 

 Crinoidea, but as it is not required to transmit the passage to the alimentary 

 canal (no prolongation of the column existing in this animal) it is without 

 central perforation. 



The PELVIS. On the margin of the pentagonal plate, rests an annular 

 plate, resembling the rim of a basin, and forming with the former a basin-like 

 cavity. It appears to occupy the place of the pelvis of the Pentacrinite, it is 

 consequently necessary that it should increase in proportion with the contained 

 viscera, without altering materially its basin-like form ; yet as it is not formed 

 of separate plates, and cannot therefore increase laterally their margins, 

 another mode has been adopted, as I shall explain in proceeding. At 

 the upper edge this pelvis-like plate is pentagonal, having between each 

 of the angles a horse-shoe-like impression for the insertion of the first costal 

 joint. Externally numerous auxiliary side arms proceed from the pelvis-like 

 plate, which when they are broken off or removed, show the exterior surface 

 of the plate marked with concave impressions (the points of their insertion) 

 each surrounded by a hexagonal rim more or less perfect, according as their 

 situation is near the central or marginal circumference of the plate. 



AUXILIARY SIDE ARMS. Across the middle of each of the concave im- 

 pressions on the pelvis-like plate, is a transverse ridge slightly perforated in 

 the centre on which the first joint of the auxiliary side arm is attached. These 

 arms, the formation of their joints, and their hook-like termination, resemble 

 in every particular those of Pentacrinus caput medusae, only that they are 

 much shorter, and formed of a less number of joints. 



GROWTH OP THB PELVIS AND AUXILIARY SIDE ARMS. It is perhaps not 



