VII. Morphology and Bionomics. 1915 265 



frame a portion of the large interradial plate that is pierced by the 

 hydropore-canal. 



In the Agelacrinidae an essentially similar peristomial frame has 

 been described, first, obscurely by Miller & Faber in a specimen of 

 Agelacrinus pileus (1892 Journ. Cincinnati Soc. Nat. Hist., vol. 15, 

 p. 85, pi. i, fig. 10), secondly, in more intelligent detail from the same 

 specimen, as well as in a specimen of Streptaster septemlrachiatus by 

 Dr. A. F. Foerste (1914, op. cit., p. 427, pi. i, fig. 5a, pi. ii, fig. 4, and 

 p. 429, pi. i, fig. 7a, pi. iv, fig. 2), who has also favoured me with 

 explanatory diagrams (in litt. , 18 Feb., 1915). In the Agelacrinidae, 

 as previously remarked (Study VI, 1915, p. 55) the floor-plates are 

 quadrangular and stretch right across the groove ; they may be 

 composed of fused pairs of floor-plates of JEdrioaster type, but direct 

 evidence for this is lacking. The proximal floor-plate in each ray 

 widens at its proximal or adcentral end, so that these five plates 

 meet, except in the posterior interradius. The adcentral borders of 

 these plates also bend down into the thecal cavity. Thus is formed 

 a roughly circular or sub-pentagonal mouth-frame, similar to that of 

 JEdrioaster and confirming the interpretation of the radial elements in 

 that genus as fused floor-plates. The gap left in the posterior inter- 

 radius is filled in by one (A. pileus) or possibly more [Streptaster) of 

 the posterior peristomial interradial plates, just as in Edrioaster. 

 For minor details reference must be made to Dr. Foerste's account ; 

 but two structural features seem to have an important bearing. 



In the specimen of Streptaster, when the mouth-frame is viewed 

 from the inner face, there is seen on the left side of the posterior 

 interradius a curved process, apparently attached to the inner face of 

 thetegmen. This separates " a vertical cavity, less than a millimeter 

 in diameter, apparently leading to the oral surface of the theca " just 

 on the posterior side of ray V, from "abroad inclined groove" leading 

 towards the thecal cavity between rays IV and V. Dr. Foerste's 

 suggestion that this groove served for the passage of the gut seems 

 entirely justified, and confirms the view that the gut had a solar coil. 

 The stereom process served, one may suppose, for the attachment of 

 one end of the posterior mesentery ; and the deep narrow passage on 

 the posterior side of it, adjoining ray V, was probably for the passage 

 of a hydropore-canal. That, at any rate, is where such a structure 

 would have lain if it existed. 



In the specimen of Agelacrinus pileus, the posterior element of the 

 mouth -frame is ridged on its inner face " somewhat like a letter W, 

 the sides of the letter abutting against the thickened inner margins 

 of the adjacent proximal floor-plates" of rays I and V. The middle 

 ridge of the W passes towards the centre of the tegmen ; and towards 

 almost the same point are directed similar ridges on the inner faces 

 of the tegminal plates between rays III & IV and III & II 

 respectively. This suggests that the large posterior plate is a com- 

 pound structure, consisting partly of an interradial element, partly of 

 portions of original paired floor-plates, and partly of the cover-plates 

 which have been transformed on the exterior into the posterior 

 tegminal. Adjoining the margin of this plate where it abuts on 

 ray V, and involving the two proximal cover-plates on the adanal side 



