THE NAUTILUS. 27 



regards the beaks, sub-trigonal and witli a small rather rounded 

 pallial sinus indicating short siphons. Both have compressed and 

 inflated species ; in both tlie epidermis in fresh specimens is con- 

 spicuous. 



Sunetta differs from such species of Meretrix (or Cytherea) as M. 

 ovum Hanley chiefly by the impressed lunule, excavated escutcheon, 

 and crenulated margin of the valves. In both the subligamentary 

 ridge is transversely striated. Allowing for the mechanical differ- 

 ences due to the diffferences in form of the cardinal margin, the hinge 

 is essentially the same, and both have the shallow pallial sinus. 



On the other hand in Tivela we have the cardinal margin and 

 also the entire basal margin of the valves of Meretrix, while the hinge 

 differs in that the subligamentary ridge or furrow is radiately 

 coarsely rugose, instead of transversely regularly striate. There are 

 flat species and inflated species as in Sunetta. 



On the southeastern coast of South America there seems to be a 

 group hitherto unrecognized which to the general characters of 

 Tivela in regard to form, pallial sinus and hinge, unites the crenulated 

 jnargin of Sunetta. To this section the name of Eutivela mav be 

 applied. 



Tivela (or Pachydesma) crassatelloides Conrad, of California, has 

 the gills small, double on each side (W-fashion) dark flesh-color ; the 

 heart is orange colored, pulsating once in ten seconds ; the mus- 

 cles are reddish, the mantle dark flesh-color with the edge waxen, 

 wrinkled transversely; the foot is hatchet-shaped, livid, with a 

 swollen, wrinkled flesh-colored basal edge, the sides below the body 

 compi"es.sed, vertically wrinkled and granulose ; the body is livid 

 flesh-color, the liver brown, the organ of Bojanus yellow; the palpi 

 are small, single and bifid on each side ; both siphons are white with 

 black dots at the end, the incurrent granulose and the excurrent 

 shorter, plain-edged and smaller; they arejoined together for nearly 

 all their length. The intestine is white and there is at the entrance 

 of the stomach a curious cartilaginous translucent hollow organ 

 through which the food must pass, difficult to describe in words, the 

 lid or upper part of which has the form (y^-^ — ■,) of a bracket, while 

 the whole is somewhat T-shaped, or perhaps vasiform. The whole 

 thing is about 6 mm. in longest diameter and resembles an internal 

 mandibular apparatus more than a gizzard ; at least its size would 

 seem to be insufficient for an organ of the latter kind. I have not 

 been able to examine specimens of the smaller Tivelas with the soft 

 parts or to get any light on the features of the soft parts of Sunetta. 



