104 CATALOGUE OF THE BLASTOLDEA. 



Schizoblastus Sai/i (PL III. figs. 1-3), and Mesoblastus crenulatus (PL IV. fig. 1 ; 

 PL VI. fig. 8), because there are ten distinctly visible openings. 



In these three genera, however, the spiracles are formed in a different way from 

 those of Pentremites Burlingtonensis, as will be explained subsequently. 



The spiracles of the genus Pent remit idea are formed in a very similar manner to 

 those of Pentremites, being sometimes divided by a septum and sometimes not. 

 Thus for example there is no septum in our specimens of P. Eifelensis, P. Gilbert- 

 soui, or P. X leda (PL V. figs. 2, 10, 13) ; while it is present in one or two of the 

 spiracles of P. clavata, P. angulata, and P. Roemeri (PL IV. figs. 14, 17; PL V. 

 fig. 15). In all the species of the genus the lancet-plate is more or less concealed 

 by the side plates which rest upon it, but project beyond it so as to fill up the gap 

 between it and the radial at its sides (Pl^IV. figs. 12, 14, 17 ; PL V. figs. 3, 4, 19). 



They thus form the distal border of the spiracle exactly as in Pentremites (PL I. 

 fig. 6 ; PL XVI. fig. 21), and when they are removed the spiracle is converted into a 

 sinus at the side of the ambulacrum, as is seen in our figures of Peutremitidea angu- 

 lata and P. Malladai (PL IV. fig. 14; PL V. fig. 19). 



Both in P. PaiUettci and in P. Lusitanica the ambulacrum is rather lower 

 in the radial sinus than in other species of the genus, and the consequence is 

 that the spiracles do not appear as large round openings, truncating, as it were, the 

 interradial portions of the calyx (PL IV. fig. 17 ; PL V. figs. 2, 15) or separated 

 from them by side plates (PL V. figs. 4, 10), as in some Pentremites (PL I. fig. 4). 

 But the whole of the oral crest is visible from the point where the radial limbs 

 terminate and the deltoids begin (PL IV. figs. S, 10, 12) ; and the spiracles are 

 minute openings at the sides of this crest which can only be seen at all in specially 

 well-preserved examples such as that figured by Roemer L A tendency to the same 

 arrangement is seen in Peutremitidea si mil is and P. Malladai (PL V. figs. 16, 19). 



Mesublastus does not differ very much from Peutremitidea in the structure of its 

 spiracles, though considerably so in the arrangement of the hydrospires. The latter 

 type resembles Pentremites in the absence of any covering to those hydrospire-slits 

 which lie at the sides of the (under) lancet-plate and are not concealed by it (PL IV. 

 figs. 12, 14; PI. V. fig. 3). But in Mesoblastus removal of the side plates does not 

 expose the hydrospire-slits, which are all concealed by the hydrospire-plate as already 

 explained (PL IV. fig. 4 ; PL VI. fig. 10 ; PL VIII. fig. 6). The deltoids are relatively 

 larger than in Peutremitidea, appearing on the exterior of the calyx (PL VI. fig. 12; 

 PL VIII. fig. 1). Their proximal ends have a tolerably wide ridge, at each side of 

 which is the flattened portion that forms the floor of the spiracle (PL IV. fig. 1; 

 PI. VI. figs. 7, 8, 13 ; PL VIII. figs. 2, 0). This is bridged over by the side plates 

 which reach from the lancet-plate to the body of the deltoid, and the tunnel thus 

 formed leads in beneath the hydrospire-plate on which the side plates rest, as shown 

 in PL VI. fig. 13. The presence of the median deltoid ridge or oral crest thus 

 causes the spiracles to be double. But it is sometimes absent (PL VIII. fig. 4), just 



-' Arcliiv f. Kufurgtscb. 1851, Juhrg. xvii. lid. i. Taf. iv. fig. 17 b. 



