150 H. L. Hmckins — Dental Apparatus of Biscoidea. 



The length of the tooth extracted for examination is 6-2 mm. 

 (measured along tlie chord), and the breadth from side to side is 

 uniformly "9 mm. 



The relative length of the symphysis as compared with the maxilla 

 in Loven's specimen was '74, whereas it is '67 in the present example. 

 That this proportion increases with the age of the individual seems 

 certain ; thus, in a series of measurements I have taken of these 

 lengths in small specimens of JiJclmius miliaris, the proportion varied 

 from -52 (in a very minute specimen, the total length of the maxilla 

 of which was only 2*3 mm.) to "61 in a specimen with a maxilla 

 3'lmm. long. But the two specimens of Biscoidea now under com- 

 parison have maxillae of precisely equal length, namely, 8-1 mm., so 

 that the difference in the relative length of the symphyses ("07) is not 

 of necessity due to difference in the age of the individuals. The 

 general proportions of the maxillae in the HQW specimen are con- 

 siderably more slender than those of Loven's specimen. 



The method of articulation with the perignathic girdle is comparable 

 to that of the Clypeastroids. The narrow adoral end of the fovea 

 magna externa is deeply depressed, and articulates perfectly with 

 a radial process of the perignathic girdle. Interradially the pyramid 

 seems to have leaned against two somewhat approximated radial 

 auriculae, and to have had very small possibilities of movement. 



As has been previously mentioned, Loven points out the striking 

 differences between the two types of perignathic girdle in Discoidea 

 cylindrica, and assigns the type with prominent auricles to the 

 cylindrical /orwi« elatior. His two figures (" Echinologica," pp. 48, 49) 

 exhibit the characters of the girdle in both types very clearl3^ He 

 shows that the tall form not only has more prominent auricles than 

 the depressed form, but that in it the characteristic interradial ridges 

 that radiate from the peristome to the sides of the test are more 

 strongly pronounced. 



The specimen of B. cylindrica here described is undoubtedly one of 

 the low or depressed forms. A comparison of the outline in Fig. 1 

 with that in Wright's monograph of the Cretaceous Echinoids referred 

 to by Loven (Wright, Pal. Soc, Cret. Ech., 1874, pi. xlvi, fig. 2a) shows 

 it to be almost identical in form with that typically depressed specimen. 

 But the characters of the perignathic girdle in my specimen (and also 

 in the companion specimen, which is identical with it in shape and 

 size) conform more or less completely to those ascribed by Loven to 

 the elevated form. Except for the fact that the auricles are not quite 

 so prominent in the case of the example from Sewell, the drawing in 

 "Echinologica" (p. 49) would serve as a representation of this specimen. 

 This correspondence in the perignathic features of the high and low 

 forms of D. cylindrica tends to break down the distinction between them. 



It has usually been stated (see Wright, 1874, p. 206) that the hemi- 

 spherical form of B. cylindrica is the young stage of the true cylindrical 

 form, but Loven suggests that there may be a specific difference 

 between the two forms. The present example seems to me to lessen 

 the probability of the existence of any fundamental distiiiction between 

 the two forms, and to give confirmation to the older belief. Loven 

 does not give the sizes of the specimens he describes, so that it is 



