262 CATALOGUE OF THE BLASTOIDEA. 



and in other genera. Owing to the direct communication of the hydrospire-slits with 

 the exterior there are no hydrospire-canals, pores, or spiracles. 



From an examination of the British Codasters we can confirm the description of 

 the ambulacra given by Billings. As there are no hydrospire-canals, there are no 

 pores ; and we doubt whether outer side plates are present in all the species. The 

 outline of the ambulacra varies but little. They are lanceolate in the British species 

 (PL XIII. figs. 1, 4), narrow and linear in C. Americ<mus\ narrow in C. Whitei 2 , 

 and slightly petaloid in C. Hindei 3 (PI. XII. fig. 4). The side plates do not cover 

 the lancet-plate entirely, but rest on its sides, leaving about a third of its width 

 uncovered. The sides of the lancet-plate are always deeply impressed for the 

 reception of the side plates (PI. XIII. figs. 8, 14). The latter vary in number 

 according to the species ; the British form possesses from six to ten on each side. 

 C. Whitei has twenty-three or twenty-four, whilst in C. pyramidatus there are 

 twenty-two. We have not been able to detect outer side plates in the British 

 species, though between every two side plates there is a large socket for the reception 

 of a so-called " pinnule." Lyon described the ambulacra of C. alternatus as " divided 

 into four equal parts by three indented lines" 4 . Shumard described and figured the 

 same thing in C. pyramidatus, and the structure of our C. Hindei 5 is identical 

 (PL XII. fig. 4). The middle one of the three indented lines is the ambulacral 

 groove, the two lateral ones bounding the side plates, which here lie upon, and 

 almost entirely conceal, the lancet-plate. The outer side plates are placed outside 

 the lateral " indented lines," and project somewhat upwards. The number of hydro- 

 spires varies in different species. In McCoy's C. trilobatus there are as many as ten 

 exposed in each triangular area, or sometimes nine exposed and one partly concealed 

 under the side of the adjacent ambulacrum (PL XIII. fig. 8). In other varieties 

 of this species we meet with all the intermediate numbers of grooves or slits, from 

 two to seven or eight, but always one, and sometimes one and a half are concealed 

 (PL XIII. figs. 1, 4, 9, 11 ; PL XVI. fig. 2). The hydrospire-slits in C. pyrami- 

 datus are six or eight in number (PL XII. fig. 1), and there are seven or more in 

 C. alternatus, var. eloncjatus, Wachsmuth, var. (PL X. fig. 19). Lastly, in C. Hindei 

 there are seven apertures on each half of the anterior interradial spaces, one of which 

 is more or less covered by the edge of the ambulacrum (PL XII. fig. 4). The inter- 

 radial or oral ridges present some marked peculiarities, and the outline of the summit 

 depends very considerably upon the flatness or the arched character of the deltoid 

 plates which bear these ridges. In G. pyramidatus they are tolerably flat, broad, and 



1 Slmmanl, Trans. St. Louis Acad. Sci. 1858, vol. i. no. 2, p. 239. 



2 Boston Journ. Nat. Hist. 1861, vol. vii. no. 2, p. 327. 



3 Shumard, Trans. St. Louis Acad. Sci. 1858, vol. i. no. 2, p. 238. 

 * D. D. Owen's 3rd Keport Geol. Survey Kentucky, 1857, p. 404. 

 5 Ann. & Mag. Nat. Hist, 1882, vol. ix. p. 235. 



