PROSOBRANCHIATA. 



785 



Fig. 686. — Macrocheilus 

 arculatus. Middle De- 

 vonian. 



Silurian to the Trias, but attains its maximum development in the 

 Carboniferous Limestone. 



The genus Macrocheilus (fig. 686) includes thick smooth shells, 

 with a moderately long spire and a pointed 

 apex, and with convex whorls. The aperture 

 is oval, not distinctly notched ; the inner lip is 

 callous, and the columella is imperforate and 

 obtusely folded in front. The genus ranges 

 from the Devonian to the Trias, the majority 

 of the known species being Carboniferous. 

 No species of the genus has been detected in 

 the later Secondary or Tertiary deposits, but a 

 living Japanese shell has been referred here. 

 The Devonian and Carboniferous genus Ortho- 

 nema appears to be related to Loxonema, and 

 the Carboniferous genus Soleniscus has been 

 placed here, though the aperture is canalicu- 

 lated in front. Lastly, the Secondary and 

 Tertiary genus Pseudo melanin finds a place in 

 this family. 



Family 28. MelaniiD/E. — In this family 

 the shell is spiral and turreted, covered with a thick epidermis, and 

 often much eroded towards the apex of the spire. The aperture is 

 oval, and either entire or channelled or 

 notched in front (fig. 687), the outer lip 

 being acute. The operculum is horny 

 and spiral. The members of this family 

 are essentially fresh-water Gastropods, 

 a few forms only inhabiting brackish 

 waters. 



In Melania itself the shell is usually 

 adorned with striae, ribs, or spines, and 

 the aperture is entire and pointed above 

 (fig. 687, a). The recent species of 

 Melania have an exceedingly wide geo- 

 graphical range, and fossil forms are 

 found in moderate numbers in the Cre- 

 taceous and Tertiary deposits. In Plen- 

 rocera the aperture is canaliculated be- 

 low, and the outer lip is sinuated. The 

 recent forms of this type are North 

 American, and the genus has numerous representatives in the 

 Cretaceous and Tertiary deposits. In Melanoflsis, lastly, the aper- 

 ture is deeply notched, and the spire is often shortened ; while in 

 Pire?ia (fig. 687, c) the aperture has the same form, but the shell 



vol. 1. 3D 



A, Melania amartila, 

 and (b) its operculum, Recent ; 

 c, Pirena atra, Recent. (After 

 Woodward ) 



