A MONOGRAPH OF THE EXISTING CRINOIDS 43 



following they are usually well circumscribed. In the not uncommon autotomy of 

 the pinnules the fracture therefore always occurs between the second and third 

 pinnulars— the articulation that comes nearest the ligamentary articulation of the 

 arms. 



The articulation between the third and fourth pinnulars, like all those following, 

 is similar to that between the second and third, but the pits for the muscular fossae 

 are better marked. The development of the muscle bundles shows great variability. 

 In the distal parts of the proximal pinnules the movement thus takes place laterally. 



In the distal articulations of the distal pinnules there is usually a pit in the dorsal 

 portion of the dorsoventral crest. This pit harbors a little dorsal ligament. Here a 

 possibility is present for slight flexibility also in a dorsoventral direction, most strongly 

 developed in certain Comasteridae, as for instance Comatulella brachiolata. 



Hyucrinidae. — In Ptilocrinus pinnatus the transverse crest of the face of the 

 pinnular forms an angle with the brachial articular face of over 150°. The calcareous 

 lamella over the muscle insertion projects forward as a large spout-shaped process. 

 The proximal joint lace of the first pinnular is peculiarly twisted in relation to the 

 distal face on the same ossicle. This last face is a straight muscular articulation 

 without interarticular ligaments. The second and following articulations are very 

 close; they possess a dorsal ligament pit, but except for this lack nearly all sculpture. 

 As is evidenced by sections these unions are in reality synostoses and therefore lack 

 muscles. 



Pentacrinidae. — The two muscle attachments on the pinnular face of the IBr 2 in 

 Metacrinus are very small and close together. They project rather a long way into 

 the brachial, and are overshadowed by a projecting calcareous lamella. The proximal 

 face of the first pinnular makes an angle between the transverse ridge and the inter- 

 muscular furrow of about 50°, this angle facing toward the median portion of the 

 brachial. The distal end of the first pinnular shows a typical straight muscular 

 articulation with the adoral parts most strongly developed. The distal end of the 

 second pinnular shows only indistinct traces of muscular attachments. As in the 

 immediately following ossicles the adoral side is the one most strongly developed. 

 The synarthroid type of articulation is very little accentuated here, the union ap- 

 proaching the synostotic. There is, however, a dorsal ligament pit. The joint faces, 

 which follow rectangularly in transverse section, have small ventral muscle pits and 

 retain the dorsal ligament pit; the ligament pits are shallow, becoming very insignifi- 

 cant distally. The dorsoventral crest is bounded dorsally by the dorsal pit. 



The articulations of the distal pinnules show the same type. The dorsal pit is 

 retained, and the dorsal ligament fossa is more distinctly accentuated. The distal 

 segments of the distal pinnules in transverse section are either rectangular (Meta- 

 crinus) or triangular (Diplocrinus) and have only a very thin calcareous bridge sepa- 

 rating the furrow for the ventral soft parts from the dorsal nerve. This thin bridge is 

 often sievelike (?for the ciliated pits) and the joint therefore acquires— as seen from 

 the ventral side— a certain likeness to the Silurian forms transitional between the 

 canaliculate and noncanaliculate types. 



As seen in transverse section the pinnulars of the Pentacrinidae, by their rectan- 

 gular or triangular form, remind one rather strongly of those found in the Thalassome- 

 tridae. 



