9G IMR. P. H. CAEPEXTER ON THE GENUS ACTINOMETRA. 



wliicli arc the remains of the upper margins of the primitive basal phites on ^hich 

 the first radials rested. Occasionally the apex of the orii^inal hasal is left unabsorbed, 

 so that the two lateral curved processes vi^hich 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 PI. 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 witli them 

 along the inner margins of their dorsal faces (fig. 16, h.b) ; and as they partially cover 

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

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

 h.b). 



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

 PL IV. fig. 16. It is remarkably well developed, being nearly as distinct as in Jctlno- 

 melra (PI. V. figs. 3, 12, and PI. VI. figs. 4, 13, 24, h.h), in which its presence is normal, 

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

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



(§ GG) 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 Actinometra and in many Antedons 

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

 plates, is uormaUy present (PI. V. figs. 3, 8, 12, and PI. VI. figs. 4, 6, 13, IS, 19, 22, 24, 

 b.h). While at the same time, as already mentioned, the interradial 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 Aiit. rosacea, and acquire a connexion witli 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 diCFerent 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 interradial, as it occupies the space enclosed between 

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

 and of the interradial furrows on the corresponding dorsal surface of the radial pentagon 

 (PI. VI. figs. 4, 13, 21., S). 



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

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

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

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

 {p), extending outwards from the base of the interradial process, and representing the 

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



