DESCRIPTIONS OF THE SPECIES. 155 



it seems to us more probable thai the canal should be in the upper or Lancet-piece, 

 as in Granatocrinus and Mesoblastua (PI. XVII. figs. 4-10), than in the lower and 

 much thinner under lancet-piece which is unrepresented in most other Blastoids '. 



Our actual knowledge of the genus Pentremites lias been gained from a limited, 

 though typical and well-defined', scries of species. We are unable therefore to give 

 so complete a general epitome of the calyx structure in this genus as we could wish, 

 and similar to what we have tried to give for other genera. Confining our remarks 

 therefore to those species which we have personallj examined we may state generally 

 that — The summit id' the calyx in Pentremites is usually broad (PI. II. figs. 1-7, 

 24-30), but that in J', elongatus it is less so (PI. II. tigs. 11, 15), and narrower still 

 in P. conoideus (PL II. figs. 1G-1S; PL III. fig. 4). The base may be very short 

 and almost truncate as in P. conoideus and /'. elongatus, rather more elongate in 

 /'. Godoni and its varieties, and still more so in 1'. piriformis (PL II. figs. 24-30), 

 but so far as we know it is never absolutely concave. The form of the basal cup 

 varies in proportion to the length of the base. It is almost flat and saucer-shaped in 

 P. conoideus (PL II. fig. 16 ; PL III. fig. 4), deeper and larger in some forms of 

 P. Godoni, more elongate in P. sulcatus, P. obesus, and P. Chester iensis, and most so 

 in P. pyriformis with its variety symmetricus, and in P. clavatus. 



The radial plates differ but little, except in the amount of vertical and transverse 

 convexity, which last reaches its greatest limit in the extreme forms of P. sulcatus. 

 They are relatively longest in P. elongatus and P. conoideus (PL II. figs. 14-17), 

 and perhaps shortest in 1'. symmeti icus (PL II. fig. 24). The sinuses are always wide 

 and subpetaloid, whilst in certain species, such as P. sulcatus, and in some forms of 

 P. Godoni, the edges are remarkably sharp and erect. 



The deltoid plates arc usually small, and more or less rhombic, although in 

 P. elongatus they become lanceolate, owing to the lengthening of the calyx ; their 

 surfaces as a rule are concave, especially at the outer ends of the plates. 



The ambulacra do not vary greatly in the more general points of detail. They 

 may be either flat, or nearly so, as in P. pyriformis, their respective halves convex, 

 as in P. Godoni, or inclined inwards towards one another, producing a highly concave 

 ambulacrum, as in /'. sulcatus. 



We are acquainted with the hydrospires of P. Godoni, in which there are from 

 three to five folds on each side (PL XVI. fig. 19); P. elongatus possesses three 

 (PL XVIII. fig. 4), P. conoideus four (fig. 6), P. sulcatus four or five (PL XVI. 

 fig. 20), and P. pyriformis from five to seven (PL XVIII. fig. 3). The spiracles are 

 single in the great majority of the species, but in P. Burlingtonensis they are double, 

 and in P. elongatus sometimes double, sometimes single (PL I. figs. 4, 5). 



The enclosure of the distal portions of the hydrospires of Pentremites within 

 the radial plates has been already described on p. 05. It causes the ambulacrum 



1 This subject is fully discussud on pp. 5U-54. 



x2 



