MAZATLAN UNIVALVES 319 
365. CaCcUM HEPTAGONUM, 2. s. 
C. (Elephantulum) t. septangulata, anguiis valde prominenti- 
bus ; annulis rotundatis confertis cincta, angulos longitudinales 
supracurrentibus ; apertura planata, extus heptagonis forma, 
intus circulari, sulco concentrico ornatd ; operculo ?.... 
Although only a fragment was found, containing a perfect 
mouth, the shell is so different from all other known species 
of the genus, that it was thought best to name it and thus 
direct attention to its existence. By its strong longitudinal 
angles, it would rank with Elephantulum ; by its concentric 
rings, (traversing these angles) with Anellum. The front view 
_ of the aperture exactly resembles an old-fashioned clock-face 
in miniature. In a flat surface with a heptagonal border is 
eut a circular hole surrounded by a ring. The discovery of 
this curious link, which was on the point. of being discarded, 
shews the importance of not rejecting fragments. The diame. 
ter of the mouth is ext. °02, int. *015. 
Hab.—Mazatlan ; 1 imperfect sp. off Spondylus ; L’pool Col. 
Tablet 1524 contains the fragment. 
Srction B. ANELLUM. 
C. t. rare elongata, juxta aperturam parum constricta, annulis 
concentricis testa adultd cinctd. 
366. CHCUM ELONGATUM, 2. Ss. 
C. (Anellui) t. elongata, tereti ; primum levi, dein annulis 
rotundatis, parum extantibus instructd, interstitiis minunis ; 
septo ungulato, apice parum élevato, subobtuso, margine lateralt 
subrecto; operculo ?.... 
? Var. SEMILEVE ; t. huc et illue annulata, huc et illue levi. 
Known from C. firmatum by the elongated form, and by the 
scarcely raised rounded rings with very small interstices ; also 
by the plug, which is ungulate, with the apex not dextral and 
rather broad. Of the Pvariety, three specimens were found, 
not perfect enough for description, but most probably distinct. 
They seem to have relations with C. liratocinctum, but shew 
no trace of longitudinal ribs. One fragment presents a few well 
marked rings between smooth spaces. Another very large 
but rubbed specimen has a sudden contraction followed by a 
