Herbert L. Hawkins — Studies on tlie Echinoidea. 345 



amount of peristomial circumference (reduced to a simple circle) 

 built up by the two areas is approximately the same, but the 

 interambulacral portion is entrenched upon by the branchial 

 incisions. The ambulacra present a semi-elliptical margin to the 

 peristome, while that of the interambulacra is more nearly semi- 

 circular. The ambulacral pores, which are not crowded nor mucli 

 displaced from their direct line, pass down near to the adradial sides 

 of the branchial incisions, and curve round with the margin of 

 their areas. 



The primordial, unpaired plate of the interambulacrum is 

 preserved as a small, triangular ossicle. The interradial sides of 

 the branchial incisions are formed of the edges of two or three 

 paired interambulacral plates, and the single plate occupies only the 

 apex of the semicircular margin of the area. Loven (" Recent Form 

 of the Echinoconidse ") recognized the persistence of the unpaired 

 plate in Plesiechinus. 1 It is interesting as showing that this 

 typically Spatango-Clypeastroid feature was developed at the very 

 outset of " Irregularity ". 



A slight groove extends along the border of the interambulacrum 

 from the branchial incision to nearly half-way across the adoral 

 surface. It is marked by the absence of large ornament, the presence 

 of guttate granules, and the development of small pits on the 

 transverse sutures. As will be seen later, this specialized region 

 coincides in position with a definite internal structure. 



The adoral surface of Plesiechinus is slightly concave, but at 

 a region nearly coincident with the position of the apices of the 

 branchial incisions a decided, though short-lived, invagination sets 

 in. Thus the basi-coronal plates at the peristomial border are 

 almost vertically disposed. All the plates of the adoral surface are 

 surprisingly thin, even in large and gerontic individuals where the 

 adapical plates are massive. 



(b) The ambulacral processes. (Seep. 344, Figs. 2, 3.) 



These structures have been partially described by Loven (loc. cit.). 

 They present an anomalous appearance when partly cleared of 

 matrix. They seem to be massive, ridge-like prominences, situated 

 obliquely to the radius across the actual adradial suture, and 

 to possess an escarpment-like adoral end, with a steep " dip-slope" 

 passing adambitally and soon merging into the general inner surface 

 of the test. They obviously spring from the ambulacrum near the 

 peristomial margin, but are quite as clearly based upon the 

 interambulacral plates for the greater part of their extent. "When 

 the preparation of the specimen is complete the explanation of this 

 peculiar anomaly is seen. The actual process is a slender, lath-like 

 projection rising from the ambulacrum near the adradial margin and 



1 Loven (Etudes, pi. xiv, figs. 124 and 125) has shown the primordial 

 interambulacral also in Holectypus and Discoides. It is almost unnecessary, 

 if not presumptuous, to confirm the accuracy of Loven's observations on 

 Echinoid structures ; but in the case of his fig. 124 (Holectypus) I have 

 a prepared specimen which agrees with his drawing in even the minutest 

 particulars. 



