MAZATLAN UNIVALVES 239 



scarcely develop the umbilical angle, the want of which in 

 Prof. Adams' description had led me to overlook the species. 

 It somewhat resembles V. Panamensis, and is known from the 

 other Mazatlan forms by the stout radiating bars, crossed by 

 two central keels, on which the shell will stand without support. 

 There is also an angle at the shoulder, and a fourth bounding 

 the large umbilical region. The shoulder angle projects so as 

 nearly to conceal the spire. Close to the suture is a swelling 

 ridge, with a depression between it and the angle. This part 

 is'wriakled, no t always so as to correspond with the bars, which 

 are variable both in number and prominence. The lip does not 

 always end with a bar. The lip is waved as in Adeorbis, but 

 -the texture is semidiaphanous, porcellanous white, as in Vitrin- 

 ella. The mean divergence, if the shoulder angle were rounded 

 off, would be about 170. Long. '02, lat. '05, by '037. 

 Hob Panama: 13 sp. in sand; C. B. Adams. MsLZ&tlan. ; 



more than 30 sp. off Chamse and Spondyli ; L'pool Col. 



Tablet 1147 contains 5 sp. exhibiting the extremes of age 

 and sculpture. 



297. ? VlTKINELLA DECUSSATA, n. S. 



? V. t. subelevatd, turbiniformi, tenui, alba, porcelland ; an- 

 fractibus iv. et dimidio, rotundatis, quarum iii. primi laeves seu 

 spiraliter striati sunt, subito in superjiciem decussatam mutan- 

 tes ; clatkris extantibus quoad xv. spiralibus, transversis plus 

 minusve distantibus, interstitiis valde impressis ; aperturd, 

 adolescents peritremati haud continuo, labio paullum in umbilico 

 reflexo ; adultd circulari, continuo ; umbilico haud parvo. 



The true position of this shell cannot be determined till the 

 animal and operculum have been examined. But for its tex- 

 ture, which is that of Vitrinella, it might rank with Liotia. It 

 is known at once by its Turbinoid shape, and by the beautiful 

 decussation caused by prominent transverse and spiral bars. 

 The large proportion of this shell which does not display the 

 characteristic sculpture would favour the supposition of its 

 being a young shell. About 30 specimens however (perfect 

 and broken) were found, of which the greater part were of 

 uniform size. The smallest specimen is '022 across. The 

 largest measures long. '04, lat. *045 by '035, div. 115. 

 Hab. Mazatlan ; very rare, off Spondylus ; L'pool Col. 



Tablet 1148 contains 4 perfect and 2 imperfect specimens, 

 shewing variations in age and sculpture. 



