74 LEPAS ANATIFERA. 



with traces of very fine lines radiating from the umbones, 

 sometimes rather plain on the basal part of the scuta. 

 Length in proportion to the breadth of the capitulum 

 variable, owing to the varying degree to which the scuta 

 and terga have their apices produced. Scuta with the 

 occludent margin either considerably curved or nearly 

 straight. The internal tooth of the right-hand scutum, 

 close to the umbo, varies in size and form, being either 

 pointed, square, or obliquely truncated on either side, or 

 it has a notch on the summit ; internal basal rim of the 

 scuta either plainly developed or nearly absent. In many 

 specimens (PI. I, fig. 1), on the scuta, or on the scuta 

 and terga, (and sometimes more on one side of the indi- 

 vidual than on the other,) a nearly straight line, running 

 diagonally across the capitulum, of slight, quadrilateral 

 depressions, of a dirty greenish colour, with the edges 

 blending away, is either conspicuously developed, or can 

 only just be discerned. These marks increase in size from 

 the umbones to the margins of the valves. There are 

 sometimes two or even three rows on the scuta. They 

 are formed by the retention of a portion of the chitine 

 membrane, which is cast off the rest of the surface ; the 

 margins of the valves are occasionally notched slightly on 

 the line of marks ; there is no difference along this line in 

 the underlying corium. Specimens both with and without 

 a barbed carina are thus characterised. Carina; the 

 interspace between the carina and the scuta and terga is 

 not wide. The carina exteriorly, is either convex and 

 smooth, or furnished with knobs or with extremely sharp, 

 long teeth (PL I, fig. 1 b) ; small specimens, with the 

 capitulum under half an inch in length, are generally most 

 strongly barbed.* Apex more or less acuminated; width 

 and thickness variable ; sides strongly furrowed. Fork 

 (fig. 1 a) generally less wide than the widest upper part 

 of the valve, with the two prongs diverging from each 



* Mr. W. Thompson found that 15 specimens, out of about 200, attached 

 to a vessel which came from New Orleans into Belfast, had their carinas 

 barbed. 



