REPOllT ON THE CRINOIDEA. 85 



It is CQncealed in the smaller specimen by the large and prominent anal tube which pro- 

 jects forwards over it. The original of Ilypononie sarsii, the " recent Cystidean," was 

 the disk of a plated Antedon, very probably of this species, Antedon midtiradiata. 



Allusion has already been made to the frequency with which these disks are met with 

 in an isolated condition ; and their resemblance to the curious Paleeozoic forms Agela- 

 crinus, Lepidodiscus, Sec, is very striking. I know that Sir Wyville Thomson had a 

 suspicion whether these problematical organisms may not have been the separated 

 disks of some one or other of the numerous PalaBOcrinoids, as suggested by Loven and 

 Lutken.^ 



Deep ambulacral grooves with strongly plated sides are also met with on the disk of 

 Actinometra strota. This species is very common at Cape York, and its disk, which was 

 also obtained in an isolated condition, may be nearly bare, or plated very completely, as 

 is shown in PL LIV. fig. 10, and PI. LV. fig. 2. The whole of the large interpabnar area 

 in which the anal tube is situated is covered with more or less scaly plates, which become 

 stouter and more granular in the neighbourhood of the subcentral anal tube. The sides 

 of the deep ambulacra are bounded by numerous smaller plates without any definite 

 arrangement. But they are strictly limited to the disk, not extending on to the arms. 

 The large size of this armature, relatively to the tentacles and the ambulacral groove proper, 

 is well shown in the cross-section represented in PI. LIV. fig. 11. Much of it extends 

 beneath the water-vessels, and corresponds to what MiiUcr called the subambulacral 

 plates of Pentacrinus^ (PI. LXIL). 



Actinometra juJcesi is another species which is common at Cape York. The large anal 

 area is often beset with numerous irregular plates, many of which bear nodules of variable 

 size (PI. LV. fig. 1). They are smaller on the base and sides of the anal tube; and 

 there are few or none in the small interpalmar spaces between the edge of the disk and 

 the circumferential ambulacra, which are themselves devoid of supporting plates, 

 though deep like those of Actinometra strota. 



Some species, both of Antedon and Actinometra, have the ventral perisome of 

 disk and arms entirely devoid of any continuous plating ; though this may be strongly 

 developed between the lower divisons of the rays, sometimes extending up to the level of 

 the third axillary. 



C. The Visceral Skeleton. 



I use the term " visceral skeleton" to denote the numerous spicules and networks of 

 limestone w'hich occur more or less plentifully in the bands of connective tissue that 

 traverse the visceral mass of the Comatulse. It also includes the more or less regular 

 plates, often quite well defined, which occur loithin the disk of Pentacrinus. They are 



1 Canad. Nat., 1869, p. 268. ^ Bau der Echinodermen, pp. 57, 58, Taf. \\. figs. 7, 9, rf. 



