MONOGRAPH OF THE EXISTING CBINOIDS. 589 



As the lateral borders of the orals are extended (this extension always in- 

 creasing in amount distally) they may maintain contact beyond the distal apex of 

 the original spherical triangle, so that from this apex there gradually arises a 

 rounded or sharp keel, as in Comactinia mcridionalis and in Hathrometra prolixa; 

 or the two sides may increase independently in such a way as to leave a narrow 

 notch between them resting on the apex of the spherical triangle, which is eventu- 

 ally filled by a plane or nearly plane deposit which, growing distally into a rounded 

 point, forms a rhombic flattened area between the everted lateral areas and touches 

 with its proximal apex the distal apex of the original spherical triangle. 



In the following species the orals appear to be spherical triangles: 



Ptilometra miilleri. 



The orals are slightly depressed in a triangular area extending from the 

 proximal border as a base to a point a greater or lesser distance below the tip as an 

 apex in 



Comactinia meridionalis. Antedon bifida. 



Comi-ssia littoralis. Antedon mediterranea. 



Antedon petasus, Antedon adriatica. 



Compsometra loveni. 



The orals are deeply depressed in the basal triangular portion, with the lateral 

 portions abruptly and strongly everted and meeting over the distal apex of the 

 former in a more or less well-marked keel, in 



fLamprometra protectus. Heliometra glaciatis. 



Leptometra celtica. Hathrometra prolixa. 



Hathrometra sarsii. 



In a triangular area extending from the proximal border as a base to a point 

 on the median line midway between the base and the tip as an apex, the orals are 

 depressed, a rhombic area including the tip and resting with its proximal apex upon 

 the distal apex of the former is flat or only slightly curved, and the sides are more 

 or less strongly everted in 



Promachocrinus keryuelensis. Anthomctra adriani. 



Though the different type of orals are, when typically developed, very distinct, 

 there is more or less intergradation between them. 



Perisomic plating of the disk. In the pentacrinoids of certain species when, 

 as a result of the rapid increase in the size of the disk, the outer edges of the orals 

 draw away from the radial circlet, exposing a constantly widening ring of perisome, 

 this perisomic ring as fast as it appears is occupied by perisomic plates which 

 form a complete pavement between the outer borders of the orals and the radial 

 circlet; but in most species this area remains unplated. These perisomic plates 

 only appear in species in which interradials are developed, and they represent a 

 distal extension, becoming smaller and more and more irregular, of the inter- 

 radial series. 



