220 ASTRALIUM. 



Genus ASTRALIUM Link, 1807. 



Shell trochiform, generally more or less flattened above or below; 

 imperforate or umbilicate; young specimens always carinated and 

 spinose at the periphery ; operculum oval or oblong, with (except in 

 Bolma) submarginal or terminal raultispiral nucleus; the last 

 whorl forming far the greater portion of the operculum, usually 

 with one or several ribs exteriorly, following the course of the spiral 

 and most elevated at the distal extremity. 



The synonyms are Calcar (Montf.) Fischer and other authors, 

 Impemtor (Montf.) Auct., Trochus, in part, of all earlier authors, 

 and Turbo, in part, Sowerby Jr. and others. 



Authors have been considerably at variance in regard to both 

 the limits and the proper designation of this genus. I have exam- 

 ined the history of every name proposed for species of the group, 

 either as generic or subgeneric, and find that none prior in date to . 

 that of Link (1807) are entitled to any standing in nomenclature. 



I am indebted to Dr. W. H. Dall for a copy of Link's description 

 of Astralivm. It was denned in the Beschreibnufj der Natarlien- 

 Sammlung der Universit'dt zu Rostock, von D. H. F. Link, Professor, 

 etc., p. 134-135, May 17, 1807 ; the genus is briefly described by 

 Link and A. deplanatum given as the first species, with a reference 

 to figures in Chemnitz, which fix the identity of deplanatum with 

 Lamarck's A. costulatum, a species of the West- Indian group. 



Those species having a turbinate form, convex base and rounded 

 periphery, such as .1. ruyosum, A. ca'latum, A. tuber, have been 

 frequently adduced by authors as supplying the connecting links 

 between Turbo and Astral him; but such resemblance as they have 

 to Turbo is to be attributed not to any real relationship, but to a 

 secondary modification which they have undergone from the stellate 

 forms of AstraMum. That this is the case is shown by the young of 

 the turbinate species, which we find to be flattened, acutely carinated 

 and spinose, precisely as in typical Astralium.. As a rule, the young 

 of species of this genus are depressed, carinated and spinose at the 

 periphery, the spines frequently being reduced in size or lost in the 

 adult ; whilst in Turbo the young are in the spinose species smoother 

 than the adult, the spines becoming always more prominent with age. 



The real connecting forms between these genera the student must 

 look to palaeontology to supplv; for the recent species which at first 

 seem to be intermediate in characters are undoubtedly descending 

 from stellate types of Astralium. 



