the Morphology of the Blastoidea. 233 



ten exposed in each area, or sometimes nine exposed and one 

 partly concealed under the side of the adjacent ambulacrum. 

 In other varieties of this species we meet with seven grooves 

 or slits ; and some may be seen with eight. In C. acutus^ 

 M'Coy, the number varies from three in the young condition 

 to four, five, or six, according to the state of growth ; but 

 invariably one and sometimes one and a half are concealed, as 

 in C. trilohatus*. The hydros pire-slits in G. pyramidatus^ 

 Shumard, are six or seven in number, and seven or more in 

 C. alternatusj Lyon. Lastly, in C. Hindei, nobis, there are 

 seven apertures on each of the interradia,! spaces, one of which 

 is more or less covered by the edge of the ambulacrum. 



We see no reason to doubt that Codaster possessed the usual 

 plated integument over the central aperture, as noticed by 

 ^\\\ Billings t; but we have not been fortunate enough to 

 discover it in any of our British specimens. The outline of 

 the ambulacra varies but little. They are lanceolate in the 

 British species, narrow and linear in C. amer'icamiSj narrow 

 in C. Whitei, and petaloid in C. jy^?yra«^^(/a^«s. 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 notched for the reception 

 of the side-phitcs. The latter vary in number according to 

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

 side. C. Whitei has twenty-three or twenty-four |, whilst in 

 C. pyt'o^t^tidatus there are twenty-two. We have not been 

 able to detect outer side plates in tiie British species; and, as 

 before mentioned, there are no pores ; but each side plate 

 carries a large socket for the reception of the appendages. 

 The structure of the ambulacra in the American Cadasters 

 appears to be somewhat different. Lyon described the ambu- 

 lacra of C. alternatus as " divided into four equal parts by 

 three indented lines." Shumard described and figured the 

 same thing in C. pyramidatus ; and the structure of our G. 

 Eindei is identical. The middle one of the three indented 

 lines is the ambulacra! groove, the two lateral ones bounding 

 the side plates, which here lie on and almost entirely conceal 

 the lancet-plate. The outer side plates are placed outside the 

 lateral " indented lines," and project somewhat upwards ; 

 pores were not observed. 



The interradial or oral ridges present some marked pecu- 



* These two so-called species appear to graduate into one another ; and 

 some doubt if they can be regarded as distinct, 

 t Loc. cit. 1870, xlix. p. 54. 

 X Boston Joiun. ^'at. Hist. 1861, vii. no. 2, p. 327. 



