192 DISCOVERY REPORTS 



genus has numerous instances of misleading comparisons. Such are, of course, un- 

 avoidable, but none the less harmful. 



The key to the subdivision of Pteraster seems to lie in the marginal spines of the 

 mouth-plates. In the subgenus Pteraster the spines of each plate form an independent 

 web and are hence most nearly like the webs of the adambulacral plates of which the 

 mouth-plates are a modification. A majority of species have the actinolateral membrane 

 narrow, a condition which may be regarded as less specialized than that in which the 

 actinolateral spines reach or even pass the margin. 



In Retaster the union of the oral spines goes a step further. A web is present between 

 the apical spines, so that all the spines of a mouth angle are co-ordinated into a single 

 webbed series — not two series as in Pteraster. In this group, species with a narrow 

 actinolateral membrane are slightly more numerous than those with a medium or broad 

 membrane. 



In Apterodon the marginal mouth spines are not webbed at all. Sometimes a short 

 abortive web unites the bases of two spines but this is in the nature of an individual 

 variation. In this group the actinolateral membrane averages wider than in the two 

 preceding subgenera. 



The keys to the species oi Pteraster which Clark (1908) and I (191 1) published served 

 their purpose but are now no longer reliable. Both Clark and I allowed too little latitude 

 of variation in the number of adambulacral spines. In the following partitioning of 

 species among three subgenera no attempt has been made to construct a key. What is 

 needed now is a critical comparison of specimens, which are not available. 



Subgenus Pteraster s.s. (including Pterasterides) 

 Pteraster, in which the series of marginal spines of each oral plate is independently 

 webbed. 



A. With abundant or sparse calcareous deposits in supradorsal membrane : 

 Pteraster militaris, type species. Bering Sea, Arctic Ocean, North Atlantic. 

 Pteraster lebruni Perrier. Falkland and Magellanic region, and neighbouring Antarctic. 

 Pteraster lebruni brachiatus Koehler. Kerguelen region ; Marion Island. 



Pteraster rugatus Sladen. Between Kerguelen and Heard Islands. 

 Pteraster trigonodon Fisher. Southern California. 

 Pteraster marsippus Fisher. Bering Sea. 



B. Deposits of supradorsal membrane absent, or not reported: 



Pteraster jordani Fisher. Pacific Coast of North America (Washington to Lower California). 



Pteraster affitiis Smith. Kerguelen; South Africa. 



Pteraster aadeatus Koehler. Davis Sea (65° S, 96° E). Possibly the same as, or a race of, affinis. 



Pteraster flabellifer Mortensen. South Africa. 



Pteraster personatiis Sladen. South-east of Ireland. 



Pteraster rediictiis Koehler, off Azores. 



Pteraster sordidiis Perrier. Off Morocco. 



Pteraster alveolatiis Perrier. Near Azores. 



