Bd. VI: 4) THE ECHINOIDEA. 61 



Meridosternata. 



Farn. Urechinidae. 

 Plexechinus Nordenskjöldi Mrtsn. 



PI. XVII. Figs. i-S. PI. XVIII. Figs. 5—12. 



Plexechinus iVcrdenskjcldi Th. Mortensen. 1905. Some new species of Echinoidea (Vidensk. Medd. 



Naturh. Foreu. Köbenhavn, p. 242). 

 — • — — 1909. Die Echinoiden d. deutschen Siidpolar-Exped. (Er- 



gebn. d. deutsch. Südpolar-Exp. XI. Zoologie. HI. 

 p. 82). 



The test is elongate, egg-shaped, only slightly flatter in front. The actinal side 

 is rather flat, somewhat deepened in front of the actinostome; the odd interambu- 

 lacrum forms a slightl)' prominent keel, which ends in an anal snout, surrounded 

 b}' a distinct fasciole. The periproct is placed rather high on the abactinal side. 

 In profile the outline of the test resembles that of PL ductus, only the anal snout 

 is somewhat more pointed. The whole of the test is covered by a dense coat of 

 short miliary spines among which the rather coarse, not very numerous, primary 

 spines stand out prominently. 



The actinostome (PL XVII Fig. 2) is somewhat before the middle; it is slightly 

 elongate, with a circle of larger plates, each bearing a pair of tubercles (spines), along 

 its border and some smaller plates within; the mouth is eccentric, nearer the poster- 

 ior edge. The anterior part of the actinostome is distinctly sunken, the mouth- 

 opening becoming thus almost vertical — • a very interesting feature, pointing towards 

 the vertically placed actinostome of the Pourtalesiaî. 



The odd interambulacrum has the labrum in contact with the second plate 

 (PI. XVII F"igs. 2, 5), not separated from it through the junction of the ambu- 

 lacral plates I. a, and V. b.,, as is the case in PL ductus and sometimes also in 

 PL hirsiitus. (In the figured specimen the ambulacral plate V. b, is abnormalh' 

 elongate.) The sternum is v^ery large, and the same holds good for the two epi- 

 sternal plates. The fasciole traverses the outer end of these two plates, encloses 

 completely the following pair of plates, which are very small, and traverses the 

 plates 5 a^ and b.. The fasciole thus comprises only three pairs of interambulacral 

 plates, while in the two other species of the genus it comprises four pairs. The 

 three following plates, a^_j and b^-g, enclose the periproct. (In PL ductus and 

 Itirsutus the first plates reaching the periproct are a,j and b,.) The two plates a- 

 and bg are not lengthened (Fig. 13). (For comparison a figure is also given of 



