THE \MIU'l,\('i;\. 61 



the ambulacra, the lancet-plate of Pentremites being completely visible, while that 

 of Granatocriniis and Mesdblastus is more or less concealed by the side plates, and so 

 is devoid of any lateral branches to its food-groove. 



In some species of Pentremites, and notably in P. sulcatUB and its allien, which 

 have very broad and petaloid ambulacra, the superficial markings of the latter are 

 somewhat different from those which are seen in P. pyriformis. This is best shown 

 in PI. I. fig. 9. The side plates are relatively short in the direction of the ray, but 

 are nearly half as wide as the lancet-plate. The sutures between them are sunk in 

 grooves which continue outwards the lateral grooves of the lancet-plate, and are very 

 markedly crenulated. A pit is often to be seen on the suture between lancet-plate 

 and side plate with a groove proceeding from it towards the pore, just as in some 

 ambulacra of P. iryriformis (PI. I. fig. 2). The ambulacra of P. Burlingtonensis and 

 of P. Godoni are somewhat intermediate in character between those of P. sulcatus 

 and of P. jiyriformis respectively. 



The best-preserved ambulacra which we have seen in Codaster have been those of 

 C. trilobatus, as shown in PI. XIII. fig. 14. The side plates are few in number, 

 though relatively large and substantial, and they rest upon the lancet-plate without 

 concealing its broad median groove, except in some cases at its distal end. The 

 suture between every two side plates is interrupted by a relatively large pit, which 

 served (we suppose) as the articulating socket for a pinnule. The same character 

 presents itself in Orophocrinus, as shown in our figures (PI. XV. figs. 7, 14). In 

 0. verus we have also detected much smaller pits upon the side plates themselves 

 with grooves proceeding from them (PI. XIII. fig. 16), just as in the proximal parts 

 of the ambulacra of Pentremites pyviformis (PL I. figs. 2, 3). 



The ambulacra of weathered specimens sometimes acquire a very singular 

 appearance, as in the cases of Mesoblastus crenulatus and M. elongatus (PI. IV. fig. 2; 

 PI. VI. figs. 10, 11), and of Codaster Ilindei (PI. XII. fig. 7). From Eoemer's figure x 

 of a well-preserved ambulacrum in the first-named species, however, we may infer 

 that it presents no essential difference from the characters of the more typical 

 Blastoids; for it shows indications of the markings which have just been noticed 

 in Pentremites pyriformis. 



D. The Outer Side Plates. 



These were discovered by Roemer 2 , who designated them as "supplemental pore- 

 plates," a name which would be an excellent one were their presence always accom- 

 panied by that of the hydrospire-pores. But the latter are often absent at the sides 



1 Archiv f. Xaturgesch. 1851, Jahrg. xviii. Bd. i. Taf. iv. fig. 15 d. 

 - Archiv f. Xaturgesch. 1851, Jahrg. xvii. Bd. i. \\ ;S35. 



