272 INVERTEBRATA OF MASSACHUSETTS. 



ODOSTOMIA EXI'GUA. 



Shell small, ovate-conical, smooth, whitish; with a single re- 

 volving line below the suture ; aperture oval; sub-umbilicated. 



FIGURE 177. 

 State Coll., No. 27. Soc. Cab., No. 2371. 



Jam mi a exigua, COUTHOUY; Bost. Journ, Nat. Hist., ii. 92, pi. 2, f. 7. 

 Rissoa rupestris, FORBES ; Ann. of Nat. Hist., ii. 107, pL 2, f. 13. 

 Turritella bisuturalis ? SAY ; Journ. Acad. Nat. Sc. y ii. 244. 



Shell small, ovate-conical, somewhat turreted, rather obtuse at 

 apex, surface smooth, light-green, under a brownish epidermis, 

 lines of growth scarcely perceptible ; whorls five or six, separated 

 by a well-defined suture ; and, in most specimens, a distinct line 

 revolves just before the suture, giving the appearance of a double 

 suture ; the lowest whorl is proportionally larger than the others, 

 and constitutes about half the length of the shell ; aperture oval, 

 outer lip sharp and simple ; pillar lip bluish-white, smooth and 

 rounded ; a transverse, white fold is formed by the turning of the 

 lip within the shell, before which it is a little raised and turned 

 outwards, producing an umbilical chink, and is extended so as to 

 form a considerable projecting angle at the lower extremity ; 

 operculum horny. Length -J- inch, breadth T \j- inch, divergence 

 28. 



First found by Mr. Couthouy, at Chelsea, near the ferry land- 

 ing, adhering to decaying wood. It has since been found in vari- 

 ous similar situations, and under the damp portions of loose 

 stones left on the shore at low tide. 



It is distinguished from O. trifidus, with which it would be confound- 

 ed without intimate examination, by its being a less slender and 

 pointed shell, the disproportion of the last whorl, and the greater con- 

 vexity of all the whorls, and the want of two or three revolving lines 

 so characteristic of O. trifidus. In the latter shell, the fold of the colu- 

 mella is oblique, and in O. exigua it is nearly transverse. It will also 

 be necessary to compare it with the next species. I should be dispos- 

 ed to regard it as the same with Turbo unidentatus, Montagu, ( Test. 

 Brit. 324) ; but as it is impossible to decide on such minute species, 



