182 CATALOGUE OF THE BLASTOIDEA. 



History. The species now placed in Mesoblastus were formerly included by us in 

 Granatocrinus, as we then understood that genus. At the time we wrote, however, 

 we found considerable difficulty in satisfactorily disposing of several species, which 

 did not in all their characters conform to our emended view of Granatocrinus. The 

 species in question were Mitra elongata, Cumberland (=Pentatrematites oblonga, 

 G. B. Sby.), Pentatrematites crenulatus, Roemer, P. angulata, G. B. Sby., and an 

 undescribed form which we now propose to call Mesoblastus Sowerbn. 



Remarks. In almost every particular but one this group agrees with Granatocrinus, 

 so far as we are acquainted with the structure of the species composing it. But in the 

 relations of its spiracles it is more closely allied to Pentremites proper, as the genus 

 is now restricted. In other words Mesoblastus has the same outline and disposition 

 of the component parts of the calyx as occur in Granatocrinus, but its spiracles are 

 constructed on a totally different plan from those of this genus. For the hydrospire- 

 canals do not open externally by piercing the deltoid plates (PI. VII. figs. 5-7), but 

 they are continued upwards over the flattened lateral portions of these plates and 

 open at the sides of their central ridge, just as in Pentremites Godoni and P. elongatus 

 (PI. I. figs. 5, 11), though this ridge is occasionally incomplete (PI. IV. fig. 1 ; 

 PI. VIII. fig. 4). In Mesoblastus, as in the other Pentremitidae, the hydrospire-cleft 

 is converted into a canal by a roof of side plates, which thus form the distal border 

 of the spiracle (PI. VI. figs. 8, 13). But the cleft is not laid open when the side 

 plates are removed, as is the case in Pentremites (PI. I. fig. 5) ; for it has an inner 

 roof formed by the hydrospire-plate, which meets the wall of the sinus and helps to 

 support the side plates (PI. IV. fig. 4 ; PL VI. fig. 10 ; PI. VIII. fig. 6). 



It will be evident from the foregoing remarks that Mesoblastus is altogether 

 different from Schizoblastus, the type which we separated from Granatocrinus in 

 1882. Schizoblastus has no hydrospire-plate, and its spiracles are minute slits between 

 the deltoids and lancet-plate, which generally fills up the radial sinus so that the side 

 plates do not project beyond its edge and roof in the hydrospire-canal (PI. XVI. 

 fig. 12). 



The calyx in Mesoblastus may be generally described as elongate, but it is least so 

 in M. crenulatus, Roemer, sp. The base is either concave, as in M. elongatus and 

 M. angulatus (PI. VIII. figs. 3, 8), or protuberant, as in M. crenulatus and M. Sowerbn, 

 nobis (PI. VI. figs. 9, 12, 14). The radial plates are of the same great relative 

 length in all the species, and the deltoids are proportionately reduced in size. The 

 ambulacra vary in width and in their position within the sinuses. They are broadest 

 in M. elongatus (PI. VIII. fig. 1), next so in M. Sowerbii (PL VI. fig. 12) and 

 M. crenulatus, and least so in the other two species (PI. VIII. fig. 7). They are 

 depressed below the edges of the sinuses in M. elongatus, but project above them in 

 all the other forms. The median grooves of the lancet-plates arc always exposed 

 for a greater or less proportion of their length (PI. IV. fig. 4 ; PI. VI. fig. 10). The 

 hydrospircs are known to us in three species only, M. angulatus, M. elongatus, and 



