REPORT ON THE CRTNOIDEA. GO 



two leading characters are — (1) the persistence of the embryonic basals which do not 

 undergo transformation into a rosette, but remain on the exterior of the calyx between 

 the centro-dorsal and the radials ; and (2) the absence of pinnules from the lowest joints 

 of the arms (PI. VI. figs. 5, 7). A third character, of no great morphological value, but 

 important from its apparent constancy, is the acorn-like shape of the centro-dorsal, and 

 the arrangement of the cirrus-sockets upon it in alternating double rows, with the ends 

 of their horseshoe-like rims projecting somewhat outwards. 



The extent of development of the basals of Atelecrinus varies with the size of the 

 individual, apparently diminishing with age as in the Pentacrinoid larvse of ordinary 

 Comatulse (PI. XIV. figs. 5-7). In the smallest specimen of Atelecrinus balanoides they 

 are wide but low pentagons which fall away very rapidly from their interradial apices to 

 the points where they meet one another beneath the radials. The middle of each basal 

 rests on the top of one of the interradial ridges at the upper end of the centro-dorsal, 

 just as the basals of Pentacrinus rest on the upper ends of the interradial ridges of the 

 stem. In older individuals, however, just as in the Antedon-larva (PI. XIV. figs. 5-7), 

 the amount of the first radials which is visible on the exterior of the calyx becomes 

 relatively less and less, and the same is the case with the basals. These are best 

 described as triangular, with their lower angles extended so as to meet those of their 

 fellows and separate the radials from the centro-dorsal by what is practically little more 

 than a line, only visible at all under specially favourable conditions of light. Each of 

 the basals, when isolated, has the form of a short triangular prism with a flattened plate- 

 like extension on each side. They are in complete lateral contact, so as to form an 

 unbroken ring on the under surface of the radial pentagon, very much as in Pentacrinus 

 alternicirrus or in Pentacrinus wyville-thomsoni. Atelecrinus cubensis has comparatively 

 large basals which are of nearly uniform height (0 - 5 mm.) all round the calyx, rising 

 very slightly at the interradial angles; while in Atelecrinus ivyvillii each basal is slightly 

 arched, with its apex interradial, and it is only in contact with the outer edge of the 

 centro-dorsal at the interbasal sutures (PI. VI. fig. 5). 



All three species agree, however, in the absence of any rosette and in the persistence 

 of the basals upon the exterior of the calyx, a feature which appears in no other recent 

 Comatula except Thaumatocrinus and the very doubtful Comaster ; while a further 

 peculiarity lies in the complete closure of the basal ring so as to separate the radials 

 altogether from the centro-dorsal. Several, if not all, fossil Comatulae have persistent 

 primary basals in the form of prismatic rods, which meet one another in the centre of the 

 under surface of the radial pentagon, and extend outwards towards its interradial angles. 

 But they do not always reach the periphery so as to appear externally between the 

 radials and the centro-dorsal, as they gradually thin out ; and there is only one described 

 form in which there is a complete ring of united basals on the exterior of the calyx. 



As regards the characters of its calyx, therefore, Atelecnnus is certainly to be 



