MOLLUSCA OF MICHIGAN— WALKER. ^07 



From alfi7iis, it differs in size, narrow umbilicus, more rounded spire and 

 more rounded base; the arrangement of the apertural lamelhe is also cjuite 

 different. Only four of the lamella) appear on the base, the others being at 

 and above the periphery. 



STROBILOPES AFFINIS Pilsbry 



Shell umbilicated; obtusely elevated; dark 

 brown; finely and closely ribbed above, ribs 

 lighter below; spire obtusely elevated; whorls 

 5^, rather narrow, higher than wide, body 

 whorl obtusely angled at the periphery, some- 

 what flattened, but rounded below, impressed 

 Fig. 121. s. arfinis. >;8.4. around the umbilicus, which is round and 



deep: aperture Innately rounded; peristome thickened, narrowly reflected; 

 about 8 short lamellae arranged in a forwardly curved radial series from the 

 axis across the base and up the outer wall of the body whorl. 

 Alt. 2^, greater diam. 2J mm. 

 Strobilops affinis, Pilsbry, Naut. VII., 56, (1893). 



Apparently of general distribution in the southern part of the state, but as 

 yet has not been reported north of the Saginaw-Grand valley. 



Its essential characteristics as compared with the other species of the 

 genus, have already been pointed out. But it may be added, that while it 

 is uniformly larger than either of the others, in contour it is intermediate 

 between them, although, as a \vhole, nearer to virgo.thsin labyrinthica. Com- 

 pared with labyrinthica, the spire is more obtusely elevated, the whorls are 

 narrower, the base more rounded, the aperture is rounded below and the 

 peripheral angle is less' pronounced. Compared with virgo the spire is not 

 so giobosely elevated; but is higher in proportion to the body whorl; whorls 

 are rather wider, the ribs stronger, the base more flattened; the peripheral 

 angle more marked; the aperture about the same in both. The arrangement 

 of the lamellae is very characteristic five only appear on the base, the others 

 being at and above the periphery. 



Genus ACANTHINULA Beck. 



Shell minute, pyramidal or giobosely turbinate, thin, brown, minutely 

 umbihcated. Epidermis raised into lamellae crossing the w^horls. Aper- 

 ture subvertical or subcircular; the lip acute, expanded tow^ard the colu- 

 mellar insertion, the margins remote. 



Section ZOOGENITES Morse. 



Shell giobosely turbinate, perforate, ornamented with oblique, cuticular 

 lamellae. 



Animal with the foot-edges prominently 

 crenulated; labial lobes large; lower tenta- 

 cles nearty obsolete. Genitalia unknown. 



Jaw arched, with numerous wide, subob- Fig. 123. Jaw of 

 solete ribs. Radula with formula 11, 6, 1, ^-harva. (Binney.) 

 6, 11. Middle teeth tricuspid, the mesocone not reaching 

 5!^/iar^pa. IfiinSejo edge of square basal plate; laterals bicuspid; marginals 

 wide with many irregular cusps. 

 49 



