THE STEM WD CALYX. 15 



type, we find thai the azygos basal is relatively small, and occupies a nearly central 

 position at the dorsal pole ; while the ether two are of considerable Length and extend 

 upwards on to the side of the calyx beneath the azygos ray (Fig. II.. A). If, as, in 

 other Blastoids, the suture between the paired basals (y, z) correspond to raj l». 

 then this must he the designation of the broad abnormal ambulacrum, and the 

 spiracle to its left must be regarded as the anal spiracle (V'vj;. II., B). Like its fellow 

 on the right of this ambulacrum (between D and E) it is described by Shumard and 

 Yandell 1 as having no median septum such as occurs in the three spiracles al the 

 sides of the anterior ambulacra. But we know too little about this remarkable type, 

 and still less about Astrocrinus, to he able to say much about the structure of the 

 ventral side. 



The extent to which the basals are developed varies very greatly among different 

 Blastoids. In Granatocrinus campanulatus they form a live-pointed star which 

 occupies the centre of the flat under-surface formed by the pentagon of radiala 

 (PL VIII. fig'. 13). In the other British species of Granatocrinus (PI. IX. figs, 3, 4, 

 13), and more especially in 6r. cUipticus (PI. VIII. fig. IS), this surface is somewhat 

 hollowed out in the centre, and the basals lie at the bottom of the concavity, as 

 seen from the dorsal side. This is also the case in ElcBacrimis (PL II. fig. 4-3); 

 while in Granatocrinus Xorwoodi the excavation reaches a considerable relative depth 

 and encloses some of the uppermost stem-joints (PI. III. fig. 1G). The basals of 

 Schizoblastus are also somewhat inconspicuous, and flat or slightly concave in general 

 aspect (PI. VI. fig. 17). Those of Mesoblastus may either be concave as in M. angu- 

 latus (PI. VIII. fig. 8), nearly flat as in M. elongatus (PL VIII. fig. 3) and M. ere- 

 nulatus (PL VI. fig. 9), or slightly produced as in M. Sowerbii (PL VI. figs. 12, 14). 



In the genus Pcntrcmitcs the basals present a very great amount of variation. 

 In P. conoidcus (PL II. figs. 16, IS, 20) and P. elongatus (PL II. figs. 14, 15) they 

 are almost as flat as in Granatocrinus campanulatus, or at any rate hut very slightly 

 convex, so that they scarcely appear at all in a side view of the calyx (PI. II. figs. 16, 

 18, 20 ; PL III. fig. 4). Those of P. Godoni are sometimes more conspicuous and just 

 visible in a side view (PL II. figs. 1-7) ; and all gradations are traceable between this 

 condition and that of the typical P. pyriformis, in which the basals form a well- 

 defined cup, the interradial angles of which may reach to more than one third the 

 distance between the dorsal and ventral poles (PL II. figs. 24-30). All the species 

 of Pentremitidea have relatively high basals, as is well seen in Plates IV. and V.; 

 and they are unusually high in the only known species of Cryptoschisma (C. Schulzi). 

 In some individuals of this type they reach a very great length (PL V. fig. 25), and 

 may take up two thirds of the entire height of the calyx, as is well shown in Boemer's 

 figure 2 . In this respect the basals resemble those of many species of Rhizocrinus, 



1 PrOC. Acad. Nat. Sri. 1'hilad. 1S56, vol. viii. p. 74. 



-' Archiv f. Xaturgesch. 1851, Jahrg. xvii. Bd. i. Taf. iv. fig. 18a. 



