320 MAZATLAN TTNIVALVES 



very large swelling i^revious to the aperture. It measures 

 long. '112, lat. (juxta aperturam) '025 — "028. 



Of the typical form about a dozen sp. were found, but most 

 of them were rubbed. They present however the different stages 

 of growth. The youngest measures Zo«^. * 06, Za#. "OOS— "017. 



The largest „ „ -103 „ -015— -023. 



A fragment (Pconspeeific) „ „ •027. 



Sab. — Mazatlan ; extremely rare, off Spondylus ; L'pool Col. 



Tablet 1525 contains 4 sp. and fragment of different ages, 

 typical form. 



Tablet 1526 contains 2 sp. Ivar. semilaeve. 



367. Cecum sttbimpeessum, n. s. 



C. (Anellum) t. elongatd, hand parvd, soUdiore ; annulis 

 creberrimis, rotundatis, hand extantibus, interstitiis parvis, 

 smpius suhimpressis ; septo mucronato ; margine laterali vix 

 concavo ; mucrone parvd, obtusd, ad dorsum sitd ; operculo 

 ? coneavo. 



Of this species only 2 adult and 6 young specimens were 

 found ; these, beiag ia different stages, have to be retained. 

 The sculpture and habit of growth resemble those of C. elonga- 

 tum ; from which it differs especially in the narrow mucro. 

 The only specimen possessing the opereidum is of aberrant 

 growth. The sutures are least impressed in the very earliest 

 stages. The smallest sp. measures long. "058, lat. '008 — '016 



The largest „ „ '115, „ -017— -03. 



jffaJ. — Mazatlan ; extremely rare, off Spond3-lus ; L'pool Col. 



Tablet 1527 contains 7 specimens in different stages of growth, 

 of which one possesses the operculum, and another is curiously 

 deformed. 



368. C^CUM FIRMATUM, C. B. Ad. (diagn. auct.) 



C. (Anelhim) t. robustd, arcuatd, satis tei^eti ; aperfurd vix 

 constrict d ; t. juniore primum Icevi ; dein anmdis circiter 

 xxiii. — xxxiii., pHmzim aeuiioribn,t, dein parum qtiadraii^i ; 

 interstitiis snhquadratis, pltis minusve distantthus, interdum 

 aperturam versus eranidis ; septo siibungulato ; apice pai'tim 

 prominente, suhdcxtrorsiun sito, obtusiore, latiore ; margine ad 

 lat us subrecto ; operculo ? . . . . 



