8 NEW FOSSIL SHELLS 



thought that there were two or three distinct species, but the varieties glide impercepti- 

 bly into each other. It appears to be by no means an uncommon shell at Petersburg. 



The form figured is a very usual one. I have but few specimens which do not 

 exhibit a decided tendency to form regular revolutions, and in some, the axis is so 

 straight that a pin may be thrust through the umbilicus from the apex to the base. 



The second figure is a specimen fractured to exhibit the appearance of the laminae. 



V. ^ FAMILY TUBICOLID.E. 



GENUS TEREDO. 

 T. CALAMUS, PL. 34. FIG. 4. 



T. testa incognita; tubo parvo, rotunda, tenui, rugoso, striis irregularibus trantversis. 

 Shell unknown; tube small, round, thin, rugose, with irregular transverse strise- 



Diam. .08 of an inch. 



Remarks. I found several specimens of this little species of Teredo, on breaking open 

 a mass of Madrepore which they had pierced. It differs from the following shell, prin- 

 cipally, in being much smaller, thinner, and more rugose. 



T. FISTULA, PL. 34. FIG. 5. 



T. testa incognita; tubo rotunda, tereti, crassissimo, turn recto turn curvato; striis irregularibus transversis raris. 

 Shell unknown; tube round, terete, very thick, sometimes straight and sometimes curved; with a few irregular 

 transverse strise. 



Diam. .50 of an inch. 



Remarks. The transverse lines of growth vary exceedingly both in size and direction. 

 The substance of the shell is very thick, the outer diameter being frequently .50, while 

 that of the aperture is about .28 of an inch. I have them, however, even thicker. 



GENUS GASTROCH.ENA. 



G. LIGULA, PL. 34. FlG. 6. 



G. Uibo incognito; testa transversd, oblanceolatd, valde inaquilatcraii, posiice' rotundatd, antice actiminatd, inflatd, con- 

 cenlrice striatd; striis parvis, irregularibus; antice hiatu maxima, valde obliquo; margine basali tortuoso; margine dorsali recto; 

 margine antico brevissimo; natibus sub-prominulis, 



Tube unknown; shell transverse, oblanceolate, very inequilateral, rounded posteriorly, acuminate anteriorly, in- 

 flated, concentrically striate; strise small, irregular; anteriorly gaping widely and very obliquely; basal margin tor- 

 tuose; dorsal margin straight; anterior margin very short; beaks somewhat prominent. 



Diam. .15 of an inch. Length .28. Breadth .12. 



Remarks. The opening at the anterior extremity commences near the dorsal margin, 

 and extends to about the middle of the base. The beaks are placed very near the ante- 

 rior end of the shell. As in most Gastroch?ena3, there is a small interior cuneiform pro- 

 cess immediately under them. 



In procuring several specimens of this shell from a piece of madrepore which they had 

 pierced, I unfortunately broke the tubes, and am therefore unable to describe them. 



This species is not unlike the G. truncata, Sowerby, a recent shell from Panama, but 

 may be distinguished by its smaller size, rounded extremity, and longer opening. It is, 



