218 ACANTHOPLEURA. 



bb. Interior of valves blue-white, with a black tract 

 behind the white-edged sinus; exterior black 

 with a light stripe on each side of the dorsum, 

 sides granulated ; girdle black, A. borbonica. 



Subgenus MESOTOMURA Pilsbry, 1893. 



Mesotomura PILS., in the Nautilus vi, p. 105 (Jan., 1893). Core- 

 phium GRAY, P. Z. S. 1847, p. 65, 68. Not Corephium BROWN, 111. 

 Conch. G. B. edit. I, 1827. 



Acanthopleura : Valves transversely engraved inside, and having 

 the sinus denticulate ; tail valve having a long, continuous insertion- 

 plate, slit only in the middle. Girdle beset with spike-like spines. 



A monotypic subgenus related to typical Acanthopleura. It is 

 remarkable for the stout spines of the girdle, the long, continuous 

 insertion-plate of the tail valve, and the finely engraved interior. 

 In the last character it can only be compared with Enoplochiton 

 niger, which also resembles the present form in having oval eyes in 

 the shell. In Mesotomura these eyes are excessively numerous, 

 exceeding those of other Acanthoids in number. 



A. ECHINATUM Barnes. PI. 47, figs. 6-17. 



Shell oval or oblong, solid, rather depressed but carinated on the 

 dorsal ridge, the lateral slopes slightly convex. Dark brown ; where 

 eroded soiled white. 



Valves strongly beaked, having a narrow, smooth and elevated 

 dorsal band, with a shallow groove on each side of it, the sides of 

 the central areas closely and deeply engraved ivith fine flexuous 

 grooves. Lateral areas but little raised, shining, having several radial 

 rows of pustules standing upon a smooth ground, which is closely 

 studded with minute eyes, or the pits left by their destruction. 

 Anterior valve having radiating, spaced rows of pustules ; posterior 

 valve having the inucro elevated and near the posterior margin. 



Interior white, unicolored or stained with reddish in the depth of 

 each valve ; under a lens it is seen to be finely, closely engraved 

 with transverse lines and elongated punctures on the forward slope 

 of the central callus, and on the jugular area (fig. 12). Slits in 

 anterior valve 8, central valves 1, posterior valve 1 ; teeth long, 

 strongly pectinated. Sinus finely denticulate in all valves. 



