THE STEM AM) CAL3 \. 35 



(PI. I. figs. 6,7; PI. XII. fig. L3) and in Orophocrinus (PL XV. fige. I. LO). [n 

 one specimen, however, we have ground away a considerable portion of the summit 



so as to expose more of the tongue-like proximal ends of the deltoids, and their 

 mutual relation seems to be very much that of the deltoids of Pentremites piriformis 

 represented in PL XII. fig. 13, viz. a close approximation near the peristome, and 

 then a divergence as if to receive the hydrospires, of which we think that we can 

 see some indications. 



The deltoids of Elceacrinus present certain peculiarities which caused them for a 

 long time to be altogether misunderstood, in spite of Roemer's excellent account of 

 the structure of the calyx in this genus 1 . They are of very large size, and form the 

 whole of the sides of the calyx, as the radial s are quite small and limited to the 

 dorsal region (PI. II. fig. 46 ; PI. XVIII. fig. 19). Each of them appears to consist of 

 three parts, a median and two lateral ones, which are separated from it by more or 

 less definite furrows and are transversely striated in correspondence with the markings 

 on the linear ambulacra. Lyon and Billings were thus led to consider the central 

 lanceolate portion as all that represented the deltoids, which, in the words of 

 Billings", " taper to a point upward ; and their sides are bevelled so as to pass under 

 the ambulacral plates, to which they are in general so closely united that the line 

 of junction is indicated only by the difference in the markings of the surface." It 

 was first pointed out by Hall 3 , however, and is now universally recognized, that the 

 transversely striated marginal areas really belong to the deltoid plates, as was 

 originally assumed by Roemer (PI. II. fig. 46). 



It is worthy of note that the large deltoid pieces of Schizoblastus Sayi often 

 exhibit a triple division of their surface which is very similar to that presented by 

 Elwacrinus. There is a lateral portion at the side of each ambulacrum which is 

 sometimes striated longitudinally and sometimes transversely, according to the stage 

 of the weathering, as in Elceacrinus (PI. XVIII. figs. 16-18). This extends from 

 the slit-like spiracle down to the radio-deltoid suture, and is sometimes continued, 

 though reduced in width, down to the radial lip. The middle portion of the plate 

 is somewhat depressed below the level of the marginal areas, and is continued 

 beyond the spiracles into the tongue-shaped process which overhangs the peristome, 

 and is marked by a small pit of variable size (PI. III. figs. 1-3). The broad end of 

 this lanceolate piece rests below upon the radio-deltoid suture ; but the central 

 depression of the interradial area is continued downwards on to the radial below and 

 terminates by a pointed extremity between the radial lips. 



We are convinced that the apparently tripartite division of the four anterior 

 interradii of Elceacrinus is merely superficial, as in the case of Schizoblastus Sayi \ 



1 Arcbiv f. Xaturgesch. 1851, Jahrg. xvii. Bd. i. pp. 376-379. 



2 Ann. & Mag. Nat. Hist. 1871, vol. vii. p. 147. 



3 Fifteenth Ann. Rep. New York State Cab. Nat. Hist. Albany, 18(32, p. 147. 



f2 



