344 



ECHIN0DERM8 OF THE BRITISH ISLES 



1. Pourtalesia A. Agass. 



Shape of test more or less cylindrical, not narrowed in the 

 anterior part. The front end vertical. Posterior interambul- 

 acrum forming a dorsal and ventral keel, the latter continuing 

 into a rather prominent anal snout, surrounded by a fasciole. 

 Periproct situated in a small depression above the anal snout. 

 The posterior paired interambulacra unite on the under side of 

 the test, thereby separating the first plates of the two posterior 

 ambulacra from the following ones. Labrum, which is usually 

 invisible from the outside and is more or less rudimentary, lying 

 wholly within the oral invagination, is wddely separated from the 

 second plate of the posterior interambulacrum (the sternum), also 

 the first ambulacral plates of the neighbouring ambulacra joining 

 in the ventral mid-Hne (Fig. 204). Apical system at the frontal 

 edge of test, disconnected. 



Two species are known from the British (and N.E. Atlantic) seas. 



Key to the species of Pourtalesia occurring in the British seas. 

 Dorsal keel overhanging periproct 1. Pourt. Jeffrey si Wyv. Th. 



,, ,, not overhanging the periproct 2. Pourt. Wandeli Mrtsn. 



1. Pourtalesia Jeffreysi Wyv. Thomson. (Fig. 205.) 

 Test broadest in the middle, somewhat narrowing tow^ards 



12 3 



Fig. 205. — Pourtalesia Jeffreysi ; naked test, from below ( 1 ) and in side view 

 (2). Specimen with the spines preserved, from the dorsal side (3). 

 Slightly reduced. (After Th. Mortensen, " Ingolf Ech., ii.) 



the front end. Under side rather conspicuously swollen ; posterior 

 interambulacrum forming a fairly conspicuous dorsal keel (not 



