MR. P. H. CARPENTER OX THE GENUS ACTLXOMETRA. 83 



The ventral aspect of the radial pentagon of Act. Solaris (PI. V. fig. 4) consists almost 

 entirely of the conjoint ventral faces of its component pieces ; the distal faces are very 

 slightly inclined to the vertical axis of the calyx, so that portions of the fossae lodging 

 the radial muscles and the interarticular ligaments hecome visihle (PL Y. fig. 4?,f, h). 

 The ridges (g{) which bound the muscular fossa? superiorly form by their apposition the 

 outer margin of the ventral surface of the pentagonal base, which is interrupted at ten 

 points, five being radial and five interradial. The former, which lie between the two 

 muscular lamellae of each radial, indicate the union of the intermuscular furrows of the 

 distal face with the ventral radial furrow occupying the median line of the superior face 

 (PL V. fig. 1, v.r.f) ; while the latter, which are at the angles of the pentagon, are the 

 outer ends of the ventral interradial furrows (v.i.f) corresponding with the sutures 

 between every two contiguous radials, the superolateral edges of which are slightly cut 

 away, so that by the apposition of every two pieces an interradial furrow is formed. 



These interradial furrows, like the radial ones, slope gently inwards towards the centre. 

 The two sets, as soon as they pass into the axial furrows on the internal faces, become 

 respectively converted into five radial and five interradial axial canals by the union with 

 one another in successive pairs of small processes extending from the intervals between 

 them towards the central calcareous network (PL V. fig. 1, c.n). These processes are 

 the central ends of ridges which are developed on the two halves of the ventral surface of 

 each first radial, between its median furrow {v.r.f) and its lateral margins. The small 

 and irregular furrows between them usually converge towards the radial or interradial 

 furrows, where they begin to descend into the corresponding axial canals; but in two 

 cases (PL V. fig. 4, x ) they are also converted into canals by the small bridge-like pro- 

 cesses above mentioned. These intermediate canals, like the normal radial and inter- 

 radial ones, are in free communication with the rest of the spaces in the calcareous net- 

 work, just as in Ant. rosacea; but the radial ones do not extend so far towards the 

 dorsal surface of the pentagonal base as in this species, as will be seen when we come to 

 study its dorsal aspect (PL Y. fig. 3). 



(§ 56) In Act. robusta (PL A*, fig. 11) this sculpturing or development of ridges and 

 furrows on the ventral faces of the first radials is carried much further than in Act. 

 Solaris. The muscular fossae (f) are also somewhat deeper, and the median radial 

 furrows which proceed inwards from the intervals between their superior margins (g^) 

 along the ventral faces of the radials are broken up very soon into a number of small 

 irregular furrows ; all converge, however, towards the centre, by the development of 

 numerous ridges of a similar nature to those rising from the lateral halves of the ventral 

 faces in Act. Solaris. 



These ridges completely obliterate all traces of any regularity in the passage of the 

 radial furrows into the central calcareous network (en), as was so marked in Act. Solaris 

 (fig. 4). The interradial furrows, too, are not particularly distinct, as the ventral surfaces 

 of the radials fall away but little towards their lateral margins. Towards the centre, 

 however, they become more distinct, and arc bridged over by processes extended from 

 the above-mentioned ridges, so that they pass downwards as canals into the system of 

 plexiform spaces occupying the central funnel of the pentagonal base. The position of 



SECOND SERIES. — ZOOLOGY, VOL. II. 12 



