24 BRITISH PALEOZOIC ASTEROZOA. 



(r) By the presence of small ventral longitudinal writhing- muscles inserted 

 between the longitudinal ridges of the arnbulacralia (Text-fig. 21, M^). 



(iii) Structures peculiar to the Palxozoic " Ophiuroidea." — The radial Avater- 

 vascular vessel does not run in an open channel as in the primitive Asterozoan or 

 the Recent Asteroidea, but in a closed canal (Text- fig, 21) formed by overgrowths of 

 the ridge upon which the ventral transverse muscles are usually inserted. AVe 

 have here the first stages of a form of specialisation which is met with very 

 frequently indeed among the Palaeozoic " Ophiuroidea." Schondorf (04) has laid 

 great stress upon this character, which he regards as of fundamental importance. 

 He has used it to aid him in establishing a new group, the " Auluroidea," which he 

 regards as distinct from the Ophiuroidea and the Asteroidea as are the Echinoidea 

 and the Crinoidea. I am unable to agree with this (see pp. 48 — 50). 



The Structure of the Arm of Aspidosoma (jraij^e, n. sp. 



The genus Aspidosoma was, until the recent work of Schondorf, placed among 

 the Asteroidea. Schondorf made very careful and exact investigations upon the 

 structure of the arm and the disc which led him to found the group " the Aulu- 

 roidea " for this and similar forms. 



Almost at once it will be noticed with respect to the form that the ambulacraliu 

 are not opposite but alternating. This character alone shows that the species is 

 not on the direct line of descent between Stenaster and Lapworthura ,t\\e next form 

 to be described (p. 25). Nevertheless, a description of it is inserted here as its 

 characteristics serve to connect the two forms. 



The cross-section given (Text-fig. 22) shows the essential relationships 

 between Aspidosoma and Stenaster. In both forms the radial water-vascular 

 vessel is enclosed in a hollow canal, the ambulacral groove is open to the exterior, 

 and the skin on the apical surface is but weakly calcified. The canal is, however, 

 in the species under description large and conspicuous. If the ambulacralia are 

 preserved in their natural position (PI. I, fig. 8) the walls of the canal may be 

 observed in oral view as conspicuous raised hemispherical ridges divided by a 

 median suture. 



If the ambulacralia have become slightly separated, the canal can readily be 

 seen. It is so deep that the inner halves of the ambulacralia appear to be concave 

 throughout their entire depth. 



This increase in size of the canal is not, in my opinion, due so much to a 

 relatively greater importance of the radial water-vascular sj^stem, as to the fact 

 that the ridge extremities act as points for the insertion of the longitudinal ventral 

 muscles. These " wriggling " muscles have now assumed great functional 

 importance, as may be judged by the large excavations on the proximal edges of 

 the ossicles. 



