36 THE VOYAGE OF H.M.S. CHALLENGEK. 



which contains mainly types of radioles similar to those of Dorocidaris and Cidaris 

 proper. 



The spines of Rhahdocidaris are of two principal types, one of which, like those 

 of R. copeoides, has also been found in the spines of a recent species ; the other, like 

 those of such recent forms as impericdis, approaches more to the fossil types of Phylla- 

 canthus orhignyana. 



Cidaris tribidoides (PI. I. figs. 2, 3, 5, 6). 



Cidarites tribidoides, Lamk., 1816, Auim. sans Vert. 

 Cidaris tribuloides, Bl., 1830, Zooph. 



There exist in several of the European museums small specimens of Cidaris, generally 

 labelled " Africa," resembling the young of PhyUacanthus verticillata, which they have 

 usually been considered. One or two such young specimens were collected by M. 

 Bouvier at Cape Verde Islands, and are now in the Jardin des Plantes. I did not 

 attempt to incorporate them in my Kevision of the Echini for want of sufficient 

 material. 



The Challenger di-edged at Bahia in 7 to 20 fathoms, and also at Fernando Noronha 

 a few specimens of a Cidaris, which at first sight would readily be mistaken for 

 PhyUacanthus verticillata (PI. I. fig. 2). On comparing, however, young PhyUacanthus 

 verticillata with the present specimens, we readily note the following striking diff"erences. 

 In PhyUacanthus verticillata the verticillations are formed by large prominent lamellae 

 (see A. Agassiz, Revis. Echini, pi. i.'' figs. 40-42), while in this species there are a few 

 larger sharper serrations rising irregularly above the general granulation of the shaft, and 

 concentrated on from three to five swellings along the length of the shaft (PI. I. fig. 3). 

 Otherwise the radioles and papillae vary in shape much as do those of Cidaris tribidoides, 

 and bear the same proportion to the diameter of the test, and the granulation is similar 

 to that of the ordinary type of Cidaris tribidoides. The test of the largest specimen 

 collected measures 17 mm. in diameter, and differs in no particular from the test of a 

 normal Cidaris tribidoides of the same size. 



Similar young specimens having thus far been observed only in localities where 

 Cidaris tribuloides occurs, I am inclined to regard them as only a variety of that 

 species. In some of the Challenger specimens, especially the smaller ones, this abnormal 

 decoration exists only in a few of the spines, the other radioles do not differ from 

 those of young specimens of a similar stage of Cidaris tribuloides. The difierences 

 in the spines of these specimens, and those thus far regarded as the normal Cidaris 

 tribuloides are not greater than differences with which we are familiar in the 

 spines of Goniocidaris tubaria for instance. In the section of Cidaris to which Cidaris 

 tribuloides and Cidaris metularia belong, no such variation of the spines was 



