22 BRITISH PALAEOZOIC ASTER OZOA. 



neither the transverse nor the longitudinal dorsal muscles could have been -well 

 developed. All these, it will be remembered, are important primitive features. 



The adambulacralia possess the usual excavations on their proximal and distal 

 faces for the insertion of the longitudinal adambulacral muscles, as well as slight 

 excavations on the dorsal faces for the attachment of the vertical muscles which 

 stretch between these plates and the ambulacralia. 



I hesitate, at present, to place this form among the true Asteroidea, as hitherto 

 it has not been found possible to identify with certainty any plate which acted as a 

 madreporite. 



True Asteroidea undoubtedly occur in the Devonian (rheinischen Grauwacke) of 

 Germany. Schondorf has given a full account of these. His figures, e. g. 62, 

 pi. xi, fig. 2, show that in these slightly older forms the ambulacralia have 

 become exactly opposite to each other, and that small ambulacral pores have been 

 developed. The chief advances made in Mesozoic and Tertiary times are : 



(«) The increase in size of the ambulacral pore due to the greater functional 

 importance of the ampulla? ; 



(h) The greater development of the transverse and longitudinal muscles between 

 the ambulacralia which makes possible greater and more vigorous movements of 

 the arms ; 



(c) Greater differentiation of the adambulacral armature ; 



((/) Elaboration of the attachment of the ambulacralia to the adambulacralia in 

 order to strengthen the arch. 



'&' 



The Progress to the " Ophiuroidea." 



It has been seen that there are relatively few modifications to be observed in 

 the evolution of the arm of the Asteroidea. Many more changes take place in the 

 forms now to be considered. 



The Structure of the Ana of Stenaster ohtiisus, Forbes. 



The genus Stenaster has hitherto been classified among the Asteroidea. The 

 following description shows that it is really a very primitive " Ophiuroid." 



(i) Primitive Structures. — That Stenaster is not far removed from the primitive 

 Asterozoa may be judged — 



(a) By the structure of the ambulacral groove (PI. I, fig. 6). The ambu- 

 lacralia are oblong, closely touching, and carry the characteristic — |-ridge. The 

 adambulacralia have the usual pentagonal shape and possess the same ornament, 

 sub-ambulacral granules and groove spines, as do other primitive forms. There can 

 be no doubt that the groove was open to the exterior. 



(/>) By the musculature. Adambulacral longitudinal muscles are well developed 



