HEMIASTER PHILIPPII. 21 



Hasslcr Expedition on the Patagonian coast. It is extremely interesting, as 

 it is the only Spatangoid, with deeply sunken ambulacra, of which we know 

 thus far anything of its development. The figures given on PL IV. f. ^-s 

 are all natural size, and show the changes the lateral ambulacra undergo 

 as they pass from Fiff. s to Fit/. 7, and from Fig. 6 to Fig. 4. When the 

 specimens reach the size of Fig. 7 the change from comparatively shallow 

 lateral ambulacra to the deep ambulacra of Fig. 6 takes place without a great 

 increase in the size of the test. The anterior ambulacra, eventually the most 

 concave {Fig. 4), are the first to show marked signs of depressions, and in 

 slightly older specimens than Fig. 7 or Fig. g they are already deeply sunken, 

 while the postei-ior lateral ambulacra are comparatively shallow. In most 

 of the specimens examined I have found large, globular, short-stemmed j^edi- 

 cellarite situated in the sunken ambulacral petals, usually the anterior pair, 

 as mentioned by Philippi. Owing to the comparatively long spines of 

 the edge of the petals, the sunken ambulacra are completely hidden by a 

 screen of spines. The changes of outline of the test are very limited, after 

 the specimens have attained the size figured on PI. IV. f. s. The outline of 

 the test is somewhat more angular and gibbous, seen from above, and perhaps 

 less conical and somewhat more flattened at the apical system. The number 

 of genital pores is variable, as we find two or three quite indifferently ; 

 the posterior pair of genital openings is always present ; if a third exists it 

 is the right anterior one, usually, but sometimes the left. In the youngest 

 specimen figured the outline of the peripetalous fasciole is nearly the same 

 as in the oldest specimen examined {Fig. 4), though its breadth becomes 

 greater with advancing age. The odd anterior ambulacrum increases but 

 little in depth and breadth with increasing size. 



The color of the spines becomes somewhat darker in older specimens ; 

 they are in small specimens {Figs. 7, s) of a dirty yellow in alcohol. 



The changes traced in this species will do much towards simplifying the 

 affinities of the many fossil species of Hemiaster and allied genera, which 

 have frequently been distinguished on very slight grounds, — the range of 

 the changes shown to be possible in one species being far greater than dis- 

 tinctions which have served as generic features among allied fossil Spatan- 

 goids. 



Hemiaster Philippii is distinguished from A. australis by its short posterior 

 lateral ambulacral petals and its narrow peripetalous fasciole. Judging from 

 the analogy of this species, H. australis and H. cavernosus may turn out to 



