20 CATALOGUE OF THE BLASTOIDEA. 



circular suture which is so often visible in Cryptoschisma SchuJzi (PI. V. figs. 25, 26). 

 But his explanation does not altogether apply to the much flatter base of Pentre- 

 mites, in which he himself says that additional modifications have taken place. 



According to Hambach 1 the articulating surface in the centre of the base is often 

 " surrounded by a triangular or clover-leaf-shaped furrow, which induced some 

 authors to suppose that the base was composed of twice three pieces ; but numerous 

 examinations of isolated pelves have convinced me of the fact that this furrow is only 

 superficial and not a separating suture " (PI. V. fig. 27). With this statement we are 

 quite in accordance, and we think that the question of supplemental basals in the 

 Blastoidea is now finally settled. It remains, however, to try and discover the real 

 nature of the structure which has been spoken of by this name. It is well described 

 by Wachsmuth 2 in Pentremites as " a tripartite plate in form of a clover-leaf, 

 occupying the central portion of the basal disc, into which it extends for some 

 distance, following its curvature. It is frequently somewhat elevated above the 

 general surface of the basals, and can be observed in most specimens with the naked 

 eye. The sutures which separate the basals by their shorter sides, in the usual way, 

 divide the lower leaves lengthwise." This last statement requires a slight modification. 

 The basals are divided asymmetrically, two of them (y, z) being larger than the third ; 

 but the sutures on the articular facet at their lower extremity divide it into three equal 

 parts, the angles at which the sutures meet being all equal (PI. V. fig. 29). Figures 

 have been published which illustrate this point ; but we do not know that any 

 explanation of it has yet been given. It results from a slight curvature in the two 

 sutures at the sides of the small basal x, as they approach the dorsal pole. An 

 indication of this is shown in Lyon's figure 3 of the base of Pentremites angularis, 

 and we give similar figures which show its occurrence in Pentremitidea Wachsmuthi 

 and in Pentremites elongatus (PI. V. figs. 8, 29). The curvature of one of these 

 sutures is also seen in the side views of Pentremitidea similis (PI. X. fig. 2), Oropho- 

 crinus stelliformis, var. campanulatus, Hamb. (PI. XVI. fig. 5), Phcenoschisma Archiaci 

 (PI. XII. fig. 10), and Cryptoschisma Schulzi. In the latter type it commences just 

 below the circular furrow, which causes the basals to appear as if they were pentagonal 

 in form (PI. V. fig. 25). We have seen the same peculiarity in Cadaster trilobatus, 

 and have no doubt of its occurrence in Stephanocrinus, Troostocrinus, and in Triccelo- 

 crinus, though the condition of our specimens has prevented our ascertaining it for our- 

 selves. In fact Meek and Worthen 4 figure the articular facet on the lower surface of 

 the basal cup in Tricoelocrinus as symmetrically divided by the three radial sutures, 



i Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1, pp. 140, 147. 

 2 Proc. Davenport Acad. Nat. Sci. 1885, vol. iv. p. 81. 



8 " Descriptions of four new species of Blastoidea from the Subcarboniferous Rocks of Kentucky." Trans. 

 St. Louis Acad. Sci. 1860, vol. i. no. 4, pi. 20. fig. 3 a. 



4 Iloport Geol. Survey Illinois, 1873, vol. v. pi. xvi. fig. 4 a. 



