GENUS CHELONOBIA. 389 



brane is attached, a slight ledge depends (fig. 1 d), which 

 narrows off towards the rostral end of the valve. I should 

 have thought that this had been a depending adductor 

 ridge, as in several species of Pyrgoma, had there not been 

 a nearly similar ledge beneath the middle part of the basal 

 margin of the tergum. The Tergum is mitre-formed, with 

 the summit more or less truncated, and with the carinal 

 margin generally more sloping than the scutal margin : near 

 the carinal margin there is a slight furrow (fig. 1 d), some- 

 times difficult to be distinguished, with the lines of growth 

 curving down to it on each side, and consequently with a 

 very slight, but variable, corresponding projection on the 

 basal margin. This furrow and slight projection, there can 

 be hardly any doubt, represent the spur, though here placed 

 close to the carinal instead of to the scutal margin, as in 

 other genera. The tergum has a small articular ridge, 

 against which the overlapping horny articular ridge of the 

 scutum abuts. There are no crests for the tergal depressor 

 muscles. Altogether the scuta and terga are very peculiar. 

 A portion dissolved in acid exhibits no tubuli. On the 

 opercular membrane there are no hairs. The rostral depressor 

 muscles of the scuta are singularly feeble, each consisting of 

 only one or two, or sometimes three or four, very thin 

 ribbon-like fasciae ; the lateral depressores of the scutum are 

 twice as strong as the rostral depressores ; and the tergal 

 depressores a little stronger than the lateral depressores. 

 All these muscles are plainly marked with transverse striae. 



Mouth. — The lab rum is not in the least bullate, which 

 character, as well as some others that follow, I specify on 

 account of the apparent affinity of Chelonobia to the Chtha- 

 malinse, as indicated by its rostrum. The crest of the labrum 

 is notched, and on each side' of the notch there is a long 

 row of teeth, which, however, are sometimes worn away. 

 The mandibles have five main teeth ; all excepting the first 

 being laterally double. The maxillae are not notched. The 

 outer maxillse are bilobed on their inner sides. 



Cirri. — The four posterior pairs, in proportion to the 

 size of the animal, are much elongated, and are remarkable 

 from the number of their short segments. The rami of the 

 first cirrus are a little unequal in length. The second 



