44 CATALOGUE OF THE BLASTOIDEA. 



proximal end bears a large median process, shaped like an elongated shield. Its 

 surface rises rapidly from the sides towards the centre, which is produced into a strong 

 ridge or keel ; and it is the most prominent part of this keel which remained behind 

 in the specimen shown on PI. I. fig. 6. The lateral margins of the apposed deltoid 

 plates are bevelled away so as to form a sort of trough which receives this shield-shaped 

 process, the interdeltoid suture being visible at the bottom of the trough. Rather 

 beyond its middle, and therefore at its deepest part, there is an opening which leads 

 into the interior of the calyx. It is shown, though less distinctly, in PL I. figs. 6 & 7 ; 

 and we have also found it in Granatocrinus elUpticus, Phcenoschisma acutum, and 

 Orophocriiws verus (PI. X. fig. 15; PL XIV. fig. 12 ; PL XV. fig. 4). It was first 

 discovered by AVachsmuth & Springer, whose description of it will be considered 

 immediately ; and as we agree with them in regarding it as " equivalent to the opening 

 at the base of the arms in Paloeocrinoids " l , we propose to call it the ambulacral 

 opening. The interdeltoid suture in our specimen of Pentremites piriformis is only 

 continued a very short distance beyond the ambulacral opening, the truncated edges 

 of the deltoids becoming less and less marked, so that the excavation between them 

 terminates at a point which corresponds to the distal end of the shield-shaped process 

 beneath the lancet-plate (PL XII. figs. 13, 14). The central ends of the inner 

 hvdrospire-folds abut against the sides of these bevelled edges of the deltoids; and 

 the latter are received in a V-shaped notch at the proximal end of a slightly concave 

 plate with a median groove, which we propose to call the " under lancet-plate." It 

 is shaped like the lancet-plate in miniature, and in its hollow there rests a low 

 prismatic ridge, which occupies the median line on the lower surface of the broad 

 lancet-plate. Its pointed proximal end comes into contact with the distal end of the 

 shield-shaped process above described, as is well shown in PL XII. fig. 14. In this 

 remarkable specimen the lancet-plate has broken across rather beyond half its length 

 from the mouth, bringing with it the whole of the two ridges on its under side and a 

 few side plates at its edge. The relations of the under lancet-plate are thus admirably 

 exposed. It is very thin, and has a faint groove in the middle of its upper surface, 

 which is slightly concave and lancet-shaped in outline. The plate is not sufficiently 

 wide, however, to come into direct contact with the grooved and sloping sides of the 

 radial sinus, except towards its distal extremity ; for it only corresponds in width to 

 the prismatic ridge on the under surface of the lancet-plate, and not to the whole of 

 the latter. Some of the hydrospire-clefts and ridges are therefore left uncovered, and 

 are visible at the sides of the under lancet-plate when the lancet-plate is removed. 

 In other species, however, the under lancet-plate is much wider, extending right 

 across the radial sinus, and completely concealing the hydrospire-apparatus (PL XII. 

 fig. 10). 



1 "Revision of the Pala^oorinoidea." Part I. (Proc. Acad. Sci. Philad. 1879, pt. 1), Explanation of 

 pi. iii. fig. 4. 



