7':astern rROViNci: — ixtkriou region species. 



265 



Fig. 282. 



Fig. 283. 



iiaiiie of II. iStrebelij Pfr. (see Fischer and Crosse, Moll. Mex. et 

 Gnat.). 



]\Ir. Morse has ^iven the following description of the interuallaminsB 

 which characterize this species: 



" The shell has been described as having one revolving tooth within 

 the aperture, and sometimes a second one, ter- 

 minating farther within the aperture. I have 

 always found this second one constant, and also 

 a ihird one, but slightly raised between these 

 two. At the base of the shell and fiir Avithin 

 the aperture are two more revolving ribs, run- 

 ning about a third of one volution. These 

 are plaiidy visible through the substance of S- ^-^yrintMca, eni.rsed. 

 the shell. A heavy columellar tooth or rib extends from a slight dis- 

 tance within the aperture, nearly one 

 volution back. This columellar tooth 

 thickens the substance of the shell in 

 the umbilical region, and causes a dis- 

 tinct fold without the shell. A most 

 singular feature is revealed in the 

 structure of the parietal lamina',. 

 With an ordinary magnifying ])ower 

 small swellings are seen at close intervals along these laminse, which, 

 when magnified four hundred diameters, are seen to be surmounted 

 with from five to ten sharj) spines, pointing toward the aperture. Tliese 

 swellings appear to coincide in number and position with the raised ribs 

 without the shell, though they are not formed at the same time, for 

 as these laminse approach the aperture they become attenuated and 

 disappear. The surface upon which these laminae rest is granulated, 

 and not smooth, as is generally the case with the interior of shells. It 

 is difficult to imagine the use of these spiny projections, unless they 

 may act in some way as points of resistance to the animal for the sup- 

 port of a very heavy shell." 



Jaw : see p. 203. 



Lingual membrane with 78 rows of 1.3-1-13 teeth each; centrals tri- 

 cuspid, central cusp very long; laterals of same shape, but bicuspid; 

 marginals low^ broad, serrated (Terr. Moll.,Y, Plate Y, Fig. O). 



Parietal laminfB of S. lahyrinthica. 



