110 ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 



the analogies of Apiocrinus and Eugenia crinus, strengthen Pourtales's view still more 

 completely. 



The occasional fusion of the upper stem-joints with the lower part of the calyx, as 

 described ahove in the Apiocrinklce, is an excellent illustration of Midler's idea respecting 

 the correspondence between the basis and the stem-joints. 



This correspondence, however, is by no means entitled to rank as a serial homology. 

 The earliest condition of the basals shows them to be five separate plates developed in a 

 spiral around the aboral ccelom of the Crinoid embryo 1 . They have distinct homologies 

 in the apical system of the other Echinoderms 2 ; while the stem-segments, surrounding 

 the aboral coelom much in the same way as the basal circlet, arc simple undivided pieces 

 from the first, and seem to be almost or quite unrepresented in the other Echinoderms. 



(vi.) The Second and Third Radlals. 



(§ 74) The second radial oi Act. poly morpha (PI. VII. fig. 2), like that of Ant. rosacea, 

 is an oval, somewhat discoidal plate, having two nearly parallel faces — one internal or 

 proximal, articulating with the first radial, the other external or distal, articulating with 

 the third radial. The internal face (fig. 2 a) closely resembles the external face of the 

 first radial (PI. VII. fig. 1 b), with which it articulates, being divided transversely by a 

 large articular ridge (*) into a dorsal and a ventral portion ; the former is entirely occupied 

 by the fossa lodging the clastic ligament (/), which is particularly deep just below the 

 opening of the central canal (c.c). Prom the ventral margin of this opening arise 

 the two ridges which bound the intermuscular furrow (f x ), and are joined near their 

 upper extremities by the transverse secondary ridges separating the large fossae (h) 

 that lodge the interarticular ligaments from those (f) lodging the flexor muscles of the 

 ray ; the latter are excavated in a pair of thin lamelke, which extend upwards from 

 the proper ventral margin of the plate, as is seen in a view of the distal face (fig. 2 b, 

 g). Besides the above-mentioned ridges and fossa?, which correspond to similar ones 

 on the distal face of the first radial, the proximal face of the second radial shows two 

 lateral processes, in which shallow fossa? (k) are excavated. These processes represent the 

 outer portions of the distal face, which is somewhat wider than the proximal one, as the 

 lateral faces are not set at right angles to the two terminal ones, but form an oblique 

 angle with the proximal face, so that the outline of the radial, when seen from the dorsal 

 or ventral side, is trapezoidal in form (fig. 2 c, d). The shallow fossae which are exca- 

 vated in these lateral faces lodge the ligamentous substance by which the second radials 

 are united with one another in pairs : the extent of this union is, as above remarked, 

 very variable in different specimens, being generally greatest where the number of arms 

 is largest (PL II. figs. 9, 11). 



The external or distal face (fig. 2 b) is much simpler in character than the proximal 

 one, as no muscles are attached to the vertical lamella? which rise from its ventral 

 margin above the articular face proper. This last is divided by a vertical ridge (*) that 



' Gotte, he. at. pp. 595, G20. 



2 P. H. Carpenter, " On the Ural and Apical Systems of the Echinoderms." Quart. Journ. Alicr. Sci. xviii. (1878) 

 pp. 371, 382. 



