LEPAS PECTINATA. 87 



much in character, narrow and broad ridges often alter- 

 nating; frequently each ridge (but more especially the 

 ridge running from the umbo to the apex of each scutum, 

 and sometimes that alone,) is covered with prominent, 

 curled, flat, calcareous spines, giving the shell an appear- 

 ance like that of many mollusca. Other specimens show 

 no trace of these calcified projections. From the thinness 

 of the valves and the depth of the furrows, the margins 

 of the valves are sinuous. Scuta: the ridge running 

 from the umbo to the apex is unusually prominent and 

 curved; it runs very close to the occludent margin, so 

 that, differently from in all the other species, only a very 

 narrow space is left between this margin and the ridge. 

 Internal teeth, under the umbones, either sharp and pro- 

 minent, or mere knobs ; sometimes that on the right side 

 is much larger than that on the left ; sometimes they are 

 nearly equal ; sometimes that on the left is scarcely distin- 

 guishable. Internal basal rim absent, or barely developed. 



Terga : these valves have a conspicuous notch to receive 

 the apex of the scuta ; the two occludent margins either 

 meet each other at a rectangle, or at a much smaller angle, 

 causing the portion thus bounded to vary much in out- 

 line, area, and degree of prominence. This at first led 

 me to think that the P. spirula of Leach, in which the 

 point is very sharp and prominent, was a distinct species ; 

 but there are so many intermediate forms, that the idea 

 must be given up. I may remark, that in all the species 

 of Lepas, the upper part of the tergum seems particularly 

 variable. The degree of acumination of the basal portion 

 of the tergum also varies ; the internal surface sometimes 

 has small crests radiating from the umbo. 



Carina, broad, within deeply concave ; edges sinuous, 

 externally sometimes strongly barbed ; narrow above the 

 fork, which latter is wider than the widest upper part of 

 the valve; prongs sharp, thin, diverging at an angle of 

 from 135° to 180°; the rim connecting the prongs not, 

 or only slightly, reflexed. 



Peduncle, narrow, shorter than the capitulum. 



