241 



3- Acasta glans Lamarck. PI. XXV, fig. 4- — 1 1 . 



Darwix, Ch., Monograph. The Balanidae, Verrucidae, etc. 1854. p. 314, pi. IX, fig. ,« — 5c 



According to Darwin, this species is characterized by parietes which on the inner sur- 

 face are quite smooth, and which have the lateral margins of each compartment inwardly 

 prominent. The basis has the edge not (or only rarely) crenated, but furnished with six 

 inwardly prominent teeth. The scutum is strongly striated longitudinally. 



The Siboga collected a small specimen of an Acasta at Station 301 (Pepela-Bay, 

 East-Coast of Rotti island), which at first I considered to be different from the known species 

 of that genus. As I did not wish to disunite the specimen, I could not examine the inner 

 structure of the compartments etc, and so doubt remained regarding its determination. I 

 received afterwards numerous specimens of an Acasta, collected in the Java-Sea by the 

 exploration-vessel "Gier": I found that it corresponded to the smaller specimen from Pepela- 

 Bay, but also, that there was sufficiënt ground for determining it as A. glans Lamarck. 

 Since Darwin's description of this species is rather short, and as it is not sufficiently illustrated 

 (the figures given on Darwin's PI. IX illustrate only the basis seen from within, the com- 

 partments seen from within, and the tergum, internal view), it has been difficult to come to a 

 conclusion — for several reasons, however, I think the one I arrived at is well-founded. 



The inner structure of the parietes is exactly as in Darwin's definition, the basis 

 shows the six inwardly prominent teeth, and the scutum is striated longitudinally. The small 

 specimen from Pepela-Bay (PI. XXV, fig. 4) came in my hands detached from the object 

 (sponge?) in which it had been embedded. But the specimens taken by the "Gier" are embedded 

 in an open and porose sponge. The greatest diameter of the basis of the specimen from Pepela- 

 Bay is 8 mm., the height of the shell 9 mm. Those taken by the "Gier" are larger : 

 diameter of the basis from 10 to 13 mm., height of the shell from 15 to 17 mm. 



The colour of the shell is dirty white to pale pink, with, in some specimens, the 

 upper part reddish, the sheath being coloured slightly darker and transversely ribbed. The 

 outer surface of the parietes shows distinct horizontal growth-ridges. In the younger specimens 

 and in that of Pepela-Bay especially, the outer surface is studded with short calcareous points, 

 irregularly distributed but disposed in transverse rows and somewhat more densely a little 

 above the basis. The basis itself is almost quite flat in some specimens, in others irregularly 

 cup-shaped, but never very deep; the central part sometimes forms a semi-globular excrescence. 



The shape of the shell is somewhat flattened laterally, the tips of the parietes are curved 

 inwardly, the orifice is medium-sized and distinctly toothed. The tips of the parietes are - - in 

 the older and larger specimens especially — often broken off. The radii have very obligue 

 summits, the free margin being somewhat undulating; they are broadest about the middle and 

 grow very narrow towards the basis-, they are distinctly striated horizontally, the striae being 

 crossed — in the larger specimens — by very faint longitudinal lines. The alae are about as broad 

 as the radii; their free lateral margin is strengthened by a longitudinal rim. Rostrum and carina 

 are broader than the lateral compartments, the carino-lateral compartments are about one-fourth 

 the width of the parietes of the lateral compartments. The radii, where they are broadest, 



113 



SIBOGA-EXPEDITIE XXXI b. ^ I 



