40 CATALOGUE OF THE BLASTOIDEA. 



the other interradii. A similar development of the anal deltoid in some specimens 

 of Troostocrinus Eeinwardti has been already noticed. 



Another variation in the condition of the deltoid is represented in PL VII. figs. 

 12 & 13, one of these plates in Granatocrinus Norwoodi being perforated by two 

 spiracular openings, instead of by one only. This is obviously due to the fact that the 

 two hydrospire-canals coming up from the radial sinus, as shown in PL VII. fig. 7, 

 do not converge sufficiently to meet one another before opening externally on the 

 surface of the deltoid. A similar slight variation, and one which is not uncommon 

 in Pentremites, is for one or two of the ambulacra to be distinctly shorter than the 

 rest, as is shown in PL II. fig. 9. It sometimes happens that the body of the radial, 

 which is thus less deeply incised than usual, is considerably flattened instead of 

 spreading outwards to its lip, as is normally the case. 



Four- and six-rayed Blastoids are occasionally to be met with. The only case of 

 the latter which we have seen occurs in a specimen of Pentremites Godoni, the dorsal 

 surface of which is very regularly hexagonal. The usual ridges extend outwards 

 over the basal cup, that corresponding to radius D being the weakest, as is normally 

 the case. But the C ridge is a bifurcating one, and its left limb passes on to a plate 

 which has all the appearance of being the body of a radial with the usual lip. It 

 is continued up the side of the calyx, between the limbs of radials C and D, without 

 being incised by any sinus, and it appears to reach right up to the summit, so as to 

 assist in forming the anal spiracle. It is of course possible that there may be a 

 partially concealed deltoid above it, as in other interradii ; and it would then almost 

 correspond to the " longitudinal piece inserted between two fork-pieces " which is 

 figured by Hambach ' in Pentremites sulcatus, except that the latter does not appear 

 to have the characters of a radial below. But we are rather inclined to consider it 

 as an unusually developed deltoid, i. e. interradial, which, instead of lying above the 

 radials, as in Cyathocrinus, is in a line with them, as the interradials are in the 

 Rhodocrinidac ; while at its lower end it takes on all the characters of a radial, except 

 that it is not incised to receive an ambulacrum. 



The tetra-radiate condition appears to be to some extent the reverse of this, — 

 viz. a radial losing its ambulacrum and appearing as a flattened plate with traces of 

 a median ridge, and a tendency to the formation of a lip just above the basi-radial 

 suture (PL II. fig. 11), so that the base of the calyx is almost normally pentagonal. 

 But there are only four ambulacra proceeding from the summit and four spiracles 

 separating them. The summit of a distorted specimen of tins kind is figured 

 in PL II. fig. 10. We have seen three examples of Granatocrinus Norwoodi 

 which present this character in very varying degrees. In two of them the dorsal 

 aspect of the calyx is toleiably normal, but the fifth radial (C) is more or less 

 undeveloped. It either only reaches halfway up the calyx, or it spreads out above 

 1 Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1, p. 154, PL n. fig. 11. 



