Herbert L. Haivkins — tifaidies on the Echinoidea. 347 



branchial incisions, but tbis is presumably for the purpose of 

 affording a safe exit for the brancbise, and has nothing to do with 

 the jaw-apparatus. But on careful investigation it is seen that the 

 invaginated rim of the primordial interambulacral plate is thickened, 

 and leads up by a concave curve to a small, rounded prominence 

 wbich lies in the median interradial line. There is, in fact, a very 

 slight ridge, extending across the projecting portion of the 

 interambulacral margin, and culminating in a central prominence 

 whose axis is considerably inclined from the vertical. At each side 

 of the central knob (which is less than one-sixteenth of the height 

 of the ambulacral processes), the " ridge" shows a slight drop, as if 

 to render the prominence more conspicuous. A very delicate, blunt 

 carina passes adorally from the knob to the margin of the peristome, 

 dividing the concave slope of the ridge into two halves. As far as 

 I have been able to determine, the adambital side of the ridge is 

 slightly undercut, so that it overhangs the interambulacrum. In this 

 respect, as in the development of a median carina, this feeble section 

 of the perignathic girdle resembles that of many Diademoida. The 

 homologies between its several parts and those of the Cretaceous 

 Holectypoida will be discussed in the forthcoming paper to which 

 reference has already been made. 



(d) The distribution of the lantern-muscles . (Fig. 5.) 



The lantern of Plesiechinus is at present unknown as regards its 

 detailed structure. That of Discoides only, among the Holectypoida, 

 has been adequately described. It is, however, quite unnecessary to 

 argue the question of its presence or absence in the Jurassic genus. 

 During the development of one of the girdles above described, I cut 

 through certain delicate Echinoid ossicles which were without doubt 

 portions of pyramids, but the refractory nature of the matrix made 

 it impossible to see any but sectional views of them. For the 

 purpose of reconstructing the musculature of the jaw-apparatus it is 

 necessary and reasonable to suppose that the lantern was essentially 

 similar to that of the Diademoida, with tendencies in the direction 

 of that of Discoides and the Clypeastroida. 



iamJb. 



amb 



iamb. 



FlG. 5. — Schematic restoration of the attachment of the lantern muscles to 

 the perignathic girdle in Plesiechinus. pr. protractors, re. retractors. 

 The hypothetical radial compass muscles are indicated by double broken 

 lines. 



There can be little doubt that the retractor muscles were attached 

 to the upper parts of the ambulacral processes, on their interradial 



