THE SESSILE BARNACLES. 241 



and B. scanden^ are known from single specimens, B. cymhifornus 



by three. 



Genus ACASTA Leach. 



1817. Acasta Leach, .Tournal de Physique, vol 85, p. 69. 

 1854. Acasta Leach, Darwin, Rlonosraph, p. 302. 



Compartments six, thin, not porous, weakly attached together and 

 to the basis; all except the carina proA'ided with radii. Base not 

 porous, calcareous, flat, or cup-shaped; form globose or acorn-shaped, 

 the basal contour rounded or oval, not produced in the carinorostral 

 axis; opercular valves and mouth parts as in Balanus; fourth cirri 

 armed with stout, short, erect spinules or recurved teeth. Living 

 embedded in sponges or ccelenterates. 



Type. — Acasta. spongltes,'^ 



Acasta is to be regarded as a group derived from Armatobalanus, 

 from which it differs by adaptations consequent upon life in sponges. 

 These adaptive features are of quite secondary value, as Darwin rec- 

 ognized; but they are nevertheless conspicuous, and have secured for 

 it recognition as a genus. So long as we recognize that Acasta is of 

 no greater taxonomic rank than the subgenera of Balanus, I agree 

 with such good authorities as Darwin, Gruvel, and Hoek that it is an 

 advantage to retain it as a genus; the more because Balanus will 

 sooner or later be dismembered to form several genera. 



The armature of teeth on the fourth cirri was first observed by 

 Darwin, and supposed to be peculiar to Acasta; but it is now known 

 that many Baloni are similar in structure. M embranohalamis has 

 entirely homologous structures, and those of Armafobalanus and the 

 Balanus trig onus group are similar, though in some forms it is the 

 third cirrus which is chiefly modified. It would not be amiss to 

 speak of Menibranohalanus as an Acasta with membranous basis, 

 just as Semibalanus is a Balanus with membranous basis. 



Darwin described nine species, of which one is known only as a 

 fossil of the British Crag. Twelve species and one subspecies have 

 been described since his monograph appeared.- The following 21 

 species are now Icnown, those represented in the United States 

 National Museum marked with an asterisk : 



*A. spongites (Poli), 1795. Europe; Mediterranean; South 

 Africa. 



A. sulcata Lamarck, 1818. Australia. 



*A. cyathus Darwin, 1854. Madeira ; West Indies. 



A. undulata Darwin, 1854. Coralline Crag, Sutton. 



^ Leach gave an unmistakable diagnosis of Acasla, but mentioned no type. Subse- 

 fjuently he described A. montar/ui (=A. spongites) . 



" Amalthca cojci Sowerby, Proc. Malac. Soc. London, vol. 8, p. 17, pi. 1, flgs. 9—11, seems 

 to be the basis of an Acasta. See Hedley, Proc. Linn, Sec. N. S. Wales, vol. 33, 1908, 

 p. 468. 



