328 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



though it is thicker than in Balanus. In Hexelasma caUistoderma 

 the basis is partially calcified and the labrum a little thickened. 

 By its completely concrescent rostrum and rostrolateral compart- 

 ments, Hexelasnw is like Pachylasma giganteum. It remains to be 

 seen whether these plates are separate in the very early sessile stage 

 of Flexelasma, as Darwin has found them to be in P. giganteum, up 

 to the time the wall attains a diameter of about 1 mm. 



Another question remaining to be worked out is the position of 

 Bathyl>alanus Hoek. 1 It is evidently no Balanid barnacle, but closely 

 related to Ilexelasma and Pachylasma, differing from Hexelasma by 

 its distinctly calcareous basis, from Pachyla^ma by the absence of 

 caudal appendages, and from both it differs by having well-developed 

 radii. None of these characters are of very great importance; yet, 

 in the present condition of our knowledge of deep-water barnacles, 

 a union of the groups would be premature. 



KEY TO THE SPECIES OF PACHYLASMA. 



a 1 . Rostrum and rostrolateral compartments completely concrescent in the 

 adult stage (distinct and united by linear sutures for a very brief period 

 after fixation) ; the other compartments having well-developed alse. Scuta 

 having growth ridges cut by a few longitudinal furrows or impressed 



lines. Mediterranean P. yiganteutn (Philippi). 



a 1 . Rostrum and rostrolateral compartments closely united by linear sutures, 



which are sometimes obliterated externally but visible inside. 

 b l . Lateral and carinolateral compartments closely united by a linear su- 

 ture ; only the carina and the lateral compartments showing alse exter- 

 nally ; sheath long; scutum having wrinkled growth-ridges cut by slight 

 radial grooves, its width less than half the length. New South Wales. 



P. aurantiacum Darwin. 2 

 6 3 . Carinolateral compartments having conspicuous alse. 



o 1 . Scuta about twice as long as wide, having sinuous or puckered growth- 

 ridges ; tergum with no spur ; carinolateral compartments much more 

 than half as wide as the lateral. Philippines. 



P. darwinianum Pilsbry. 



r a . Scuta having sculpture of smooth, even, transverse growth-ridges, not 

 cut by longitudinal grooves or strife ; carinolateral compartment half 

 as wide as the lateral ; tergum having a very short, wide spur. 

 d\ Scutum convex, its width contained two and one-half times in its 

 length ; tergum with no longitudinal depression externally, its ar- 

 ticular ridge not projecting beyond the scutal border. China Sea. 



P. chinense Pilsbry. 



d 2 . Width of scutum contained twice in its length ; tergum having a lon- 

 gitudinal depression externally, its articular ridge projecting a 

 little beyond the scutal border. Japan P. crinoidophilum Pilsbry. 



1 Bathy-Balanus Hoek, Si&oj;a-Expeditie, Monographic 31&, p. 160, monotype Balanus 

 pcntacrini Hoek. 



2 P. aurantiacum is not contained in the United States National Museum. 



