150 CATALOGUE OF THE BLASTOLDEA. 



always more or less traceable in Mesoblastus (PL IV. fig. 1 ; PI. VI. fig. 13), some- 

 times indeed rather prominently so (PI. VI. figs. 7, 8). But in all cases alike the 

 hydrospire-canal is bridged over by side plates, whether they rest against the lancet- 

 plate as in Pentremites, or partially cover it as in Mesoblastus and Pentremitidea. 



The latter genus passes very gradually into Pentremites, especially as regards the 

 external appearance of the deltoids. They are very small in Pentremites annularis 

 and P. calycinus, and also in some forms of P. piriformis (PL II. figs. 29, 30), from 

 which last it is but a step to certain species of Pentremitidea (PL V. figs. 6, 7, 16) ; 

 while in Pentremitidea % Jeda, Hall, sp. (PL V. figs. 12, 13), they are as large as in 

 many true Pentremites (PL II. figs. 19-23), though the linear shape of the ambulacra 

 and the relation of the side plates to the lancet-plate seem to fix the generic position 

 of this species without any difficulty. 



The mutual relations of the three genera of the Pentremitidae are shown in the 

 following scheme : — 



A. Ambulacra usually broad and petaloid ; the lancet-") 



plate not covered by side plates, which only rest>Pentremites, Say. 

 against its edge. J 



B. Ambulacra usually narrow and linear ; the lancet-plate 



more or less concealed by the side plates which rest 

 upon it. 



(1) Base generally elongated. Deltoids usually") 



invisible upon the exterior of the calyx. No >Pentremitidea, d'Ovbigny. 

 hydrospire-plate. J 



(2) Base inconspicuous and somewhat flattened.") 



Deltoids appear externally. An hydrospire- >MesohIastus, E. & C. 

 plate. J 



In the following descriptions we use the word " periphery " to denote the level 

 at which the horizontal diameter of the calyx is greatest; while "summit" denotes 

 that portion of the surface of the calyx which is included between the proximal ends 

 of those parts of the deltoid plates which appear externally above the radial limbs, 

 or between the ends of these limbs themselves when the deltoids do not appear 

 externally. 



Genus PENTREMITES, 6'aij, 1820 (emend. E. & C, 1886). 



Pentremites, Say, American Journ. Sci. 1820, vol. ii. p. 36. 



Pentremites, Say, Journ. Acad. Nat. Sci. Philad. 1825, vol. iv. pt. 2, p. 292. 



