PLEKOCHEILUS. 63 



iii, pp. 114, 115 (jaws and teeth of auris-sileni, aulaco stylus. SEM- 

 PER, Reisen ira Archip. Phil, Landmoll., p. 150 (soft anatomy of 

 loveni and blainvilleanus). 



Shell " Bulimoid," ovate or ovate-acuminate, usually solid and 

 opaque ; zig-zag streaked or dappled with brown on a lighter ground, 

 or variegated with whitish over brown or yellow, rarely unicolored; 

 the surface wrinkled, striated or granulose. Nepionic shell small. 

 Aperture ovate, the lip reflexed, expanded, or blunt ; columella 

 reflexed or narrow, with a spiral fold or none. 



Left mantle-edge with two anterior lobes ; kidney very short, 

 triangular. Genitalia simple, the spermatheca duct long, penis 

 bearing a long flagellum. Jaw vertically striated by the edges of 

 numerous narrow, imbricating, slightly oblique united plates. Rad- 

 ula with the rhachidian tooth bearing a long middle cusp only, or 

 middle cusp shorter, and side cusps developed ; lateral and marginal 

 teeth with two cusps. 



Distribution, northwestern South America (British Guyana, Ven- 

 ezuela, Colombia, Ecuador and Peru, with adjacent portion of Bol- 

 ivia) ; Islands of St. Vincent and St. Lucia. 



This group has hitherto been incorporated in the restricted genus 

 Bidimus of authors, with Borus, Strophocheilus, Orphmis&ud Dryp- 

 tus ; but these groups I have shown to possess the common character 

 of a relatively very large nepionic shell. In other words, the embryo 

 undergoes a greater amount of development within the egg, the lat- 

 ter containing a considerable quantity of nutriment. In conse- 

 quence of this, the individual eggs are of large size and few in num- 

 ber. On the other hand, Plekocheilus and the genera which will 

 follow, reproduce by smaller eggs, the young at the time of hatch- 

 ing being relatively minute. The nature of the jaw also separates this 

 genus from Borus and its allies ; the latter having a sojid, ribbed or 

 smooth jaw like the Helices, while Plekocheilus has a jaw of the 

 finely laminated type common in Bulimulidce. 



The Martensian classification is, therefore, clearly erroneous. In 

 transferring the genus from Helicidce to Bulimulidce, attention may 

 be directed to the structure of the jaw, which is like many species of 

 Otostomus ; the teeth and genitalia which resemble those of some 

 Bidimulm; while the shell is either more solid than in those genera, 

 or its reflexed lip offers a differential character. 



