96 MR. P. H. CAEPENTER ON THE GENUS ACTINOMETRA. 



which are the remains of the upper margins of the primitive hasal plates on which 

 the first radials rested. Occasionally the apex of the original hasal is left unabsorbed, 

 so that the two lateral curved processes which remain after the removal of the primary 

 external layer along the median line of each "plate remain in connexion with one another; 

 as is seen in the bottom part of the rosette represented in PL IV. fig. 13. The trian- 

 gular iuterradial process (o'), which is developed from a secondary calcareous deposit 

 on the ventral side of the original basal, has here become more or less completely united 

 with these primary bars connecting the two lateral portions of the basal. The latter 

 retain their primitive relation to the first radials, for they remain united with them 

 along the inner margins of their dorsal faces (fig. 10, b.b) ; and as they partially cover 

 in the secondary basal cords, X u X 2 , &c., on their dorsal aspect before they enter the 

 central canals of the first radials, I will call them the " basal bridge " (PL IV. fig. 13, 

 b.b). 



This basal bridge is well seen in situ in the specimen of Ant. rosacea represented in 

 PL IV. fig. 10. It is remarkably well developed, being nearly as distinct as in Actino- 

 melra (PL V. figs. 3, 12, and PL VI. figs. 1, 13, 21, b.b), in which its presence is normal, 

 and not al (normal, as in Ant. rosacea. It is also slightly developed in the specimen of 

 Ant. celtica represented in PL IV. fig. 3 ; but in fig. 7 no trace of it is visible. 



(§ G6) This tendency to an incomplete metamorphosis of the embryonic basals of Ant. 

 rosacea, and consequently to the abnormal persistence of a more embryonic condition 

 than usual, is of considerable interest, because in Actmometra and in many Anledons 

 a basal bridge, representing the apex and unabsorbed margins of the embryonic basal 

 plates, is normally present (PL V. figs. 3, 8, 12, and PL VI. figs. 4, G, 13, 18, 19, 22, 24, 

 b.b). While at the same time, as already mentioned, the iuterradial processes of the 

 rosette (o), which are developed from a secondary or ventral layer, are large and spout-like, 

 as is abnormally the case in Ant. rosacea, and acquire a connexion with the remains of the 

 primary or dorsal layer which forms the basal bridge. The complicated rosette thus 

 constituted also becomes united with the large, more or less spindle-shaped, rays of the 

 basal star (S), the origin of which, as will be subsequently seen, is totally different from 

 that of the rosette. 



A single " compound basal," as it may be called, of Actinometra, thus consists of two 

 distinct elements — (i) the incompletely metamorphosed embryonic basal, and (ii) a single 

 ray of the basal star. Its position is iuterradial, as it occupies the space enclosed between 

 the apposed edges of the basal furrows on the ventral surface of the centrodorsal piece, 

 and of the iuterradial furrows ou the corresponding dorsal surface of the radial pentagon 

 (PL VI. figs. 4, 13, 24,#). 



An isolated compound basal which is thus constituted, when seen from its dorsal side 

 (PL V. fig. 8 b, PL VI. fig. 22 b), shows : — (i) more or less of the calcareous network {cm) 

 which unites the ventral surface of the rosette to the internal faces of the first radials ; 

 (ii) a large iuterradial spout-shaped process (o) ; (iii) two small, radial, curved processes 

 (//), extending outwards from the base of the iuterradial process, and representing the 

 unabsorbed lateral portions of the primary layer forming the embryonic basal plate. 



