204 ARTHUR DENDT. 



with the dermal surface^ and it is the apical ray which is 

 directed inwards through the chamber layer. Hence the posi- 

 tion of these spicules is quite different from that of the sub- 

 dermal sagittal triradiates of the Heteropidse, while in the one 

 case it is the basal ray in the other it is the apical one which plays 

 such an important part in the support of the chamber layer. ^ 



The anatomy of Heteropegma nodus-gordii is repre- 

 sented in fig. 20. It will be seen that the dermal cortex is 

 strongly developed, and has the ordinary Grantia structure, 

 except that there are no oxeote spicules. The gastral cortex 

 is extremely thin and its skeleton greatly reduced, consisting 

 of a number of very small quadriradiates arranged in the usual 

 manner. The articulate tubar skeleton is also very much re- 

 duced, consisting of very minute spicules, chiefly quadriradiate. 

 This reduction in the skeleton of the chamber layer is partly 

 compensated for by the presence of very large subdermal quadri- 

 radiates, whose facial rays are extended in the inner part 

 of the dermal cortex, while the apical ray penetrates through 

 the chamber layer almost to the gastral surface. 



The skeleton of the Victorian Heteropegma latitubu- 

 lata (4) resembles thatof H. nodus-gordii, except in certain 

 very minute details of spiculation, the tubar and gastral spicules 

 being still further reduced in size. 



Amphoriscus. 

 In this genus the articulate tubar skeleton, which was still 

 clearly recognisable in Heteropegma, has disappeared, 

 although the radial Syconoid character of the canal system 

 remains. In one species, however (Polejaeff^s Amphoriscus 

 elongatus [8]), the subgastral sagittal triradiates still persist. 

 We always find, as in Heteropegma, a layer of subdermal 

 quadriradiates with inwardly directed apical rays ; and we also 

 find, in addition to these, in nearly all species, a layer of sub- 

 gastral quadriradiates with outwardly directed apical rays.^ 



1 The subdermal quadriradiates are probably derived from the ordinary 

 triradiates of the dermal cortex by the development of an apical ray. Com- 

 pare my remarks on the skeleton of Lelapia. 



* These spicules are not homologous with the subgastral quadriradiates of 



