PILEMATECHINUS RATHBUNI. 167 



what sharper curve at the ambitus. The posterior extremity slopes at a 

 sharper angle (Pis. 82, fig. 1 ; 84, fig. ..'), and is truncated at the anal system. 



Seen facing the anal system and odd posterior interambulacrum (Pis. 82, 

 fig. 1 ; 84, fig. 2) the ambital curve is sharper on the sides of the test than 

 at the anterior and posterior extremities, and the slope of the sides of the 

 test much steeper than those of the extremities. 



The extended apical system, which is also at the apex, Figs. 238, 239, 

 occupies about the central part of the abactinal part of the test (PI. 83, 

 fig. ..'). The apical system of P. Rathbuni (Pis. 83, fig. 3; 85, figs. 3, $), 

 greatly resembles that of Urechirws Naresianus, Fig. 227, its anterior genital 

 plates are distinct, and not- united as in C. Wyvillii and C. Loveni, but the 

 madreporic body extends over both ami part of the adjoining anterior ocular 

 plates. The posterior zones of the lateral anterior interambulacra unite be- 

 tween the anterior genitals, and the anterior lateral oculars, Fig. 239, which 

 are thus isolated from the odd anterior ambulacrum, as in Urechinus, Fig. 224, 

 and do not adjoin the anterior genitals as in ('. Wyvillii and C. Loveni. 



Between the trivium and biviurn the posterior genitals are placed diagon- 

 ally, so that the anterior zone of the left posterior interambulacrum separates 

 the posterior genital from the anterior ocular, while the posterior zone of the 

 corresponding interambulacrum separates the posterior genital from the 

 posterior ocular. 



There are, however, important variations in the apical system of Pilemate- 

 chinus Rathbuni, the specimen of 95 mm., Fig. 238, having an apical system 

 much like that of Cystechinus as here limited, while in the smaller specimen, 

 Fig. 239, the arrangement of the apical plates is more like that of U. gigan- 

 teus, Fig. 224. 



The anal system is large, longitudinally elliptical (Pis. 82, fig. .„> ; 84, fig. 

 / ; 86, fig. 5), placed well above the ambitus ; while in C. Wyvillii, C. Loveni, 

 and C. vesica the anal system is on the acti- 

 nal side of the ambitus. It is covered by 

 an outer row of large irregularly pentagonal 

 plates, largest at the abactinal edge. 



The miliaries of the test all carry minute 

 spines. The few primary spines left on the 

 test were slender, cylindrical, rather short. 

 Small, short-headed pedicellarice were found scattered irregularly among the 

 miliary spines. 



85 mm. 



Fig. 240. Pilematechinus Rathbuni. 



