MUSEUM OF COMPARATIVE ZOOLOGY. If)? 



In none of the specimens are the cirrhi pi-eserved entire. They 

 ure long and slender, consisting of 30+ joints, the lowest of which are 

 quite short ; but from the hfth or sixth onwards they are very long 

 (2L mm.) and tolerably equal (Fig. 1). 



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

 individual, apparently diminishing with age as in ordinary Comatidce. 

 In the smallest specimen 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 (Figs. 4, 5), just as the basals of Pentacrinus rest on the upper 

 ends of the interradial ridges of the stem. The pentagonal shape of the 

 basals is still traceable in the slightly older specimens which are the origi- 

 nals of Figs. 1 and 3 ; but in still older ones, just as in the Pentacri- 

 noid of Antedon rosacea, 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 ai-e best described as triangular, with 

 their lower angles extended so as just to meet those of their fellows and 

 separate the radials from the centrodorsal by what is practically little 

 more than a line, only visible at all under specially favorable conditions 

 of light. In fact, I believe that even this is absent in parts of some of 

 the specimens, the radials coming into partial contact with the centro- 

 dorsal just as in Pent, aster ia. 



The acorn-shaped centrodorsal of Atelecrinus balanoides is neaidy as 

 deep as it is high. The opening of its cavity has a narrow pentagonal 

 rim, from the interradial angles of which strong ridges descend the 

 sloping walls, diminishing in size as they approach the apex, where they 

 die away without meeting one another (Fig. 5). The large openings of 

 the cirrhus sockets are visible between them. Owing to the manner in 

 which they project inwards, the centrodorsal cavity has a five-lobed 

 shape, the re-entering angles between the broad but short lobes corre- 

 sponding to the interradial ridges. The large upper ends of these ridges 

 are somewhat hollowed, as are the lower surfaces of the basal plates 

 which rest upon them (Fig. 4). When seen edgeways each of these 

 plates has the form of a short triangular prism, with a flattened plate- 

 like extension on each side (Figs. 6 a, 6). They are in complete con- 

 tact laterally, so as to form an unbroken ring around the central opening 

 of the calyx, which is not quite so large as is shown in Fig. 4 ; for it is 

 encroached upon by excessively delicate processes that project inwards 

 from near the lateral margin of each basal. Owing: to their extreme 



