NO. 2 PYRAMIDELLID MOLLUSKS — BARTSCH 57 



bridization has caused an enormous efflorescence of individuals, and 

 on the west coast it has also caused the pyramidellids mentioned to 

 extend their range over the adjacent faunal areas to which the fixed 

 species of the region are confined. 



The present complex combines features characterizing the sub- 

 genera Strioturbonilla and Pyrgiscus, and, if we follow the precept 

 of our botanists, may be called Striopyrgus, with the specific designa- 

 tion hybridus. 



It will be interesting to see if, when the Miocene pyramideUids of 

 the region are worked up, we may be able to discover the parents 

 responsible for this group of mutants, and it will be equally interest- 

 ing to note the Pleistocene and Recent descendants of the group. 



STRIOPYRGUS, new pseudogenus 



Shell of typical Turbonilla shape. The sculpture consists of axial 

 ribs and a mixture of strong incised spiral grooves and fine spiral 

 lines in the intercostal spaces. 



Type : Striopyrgus hybridus, new species. 



STRIOPYRGUS HYBRIDUS, new species 

 Plates 10-14, figures 1-61 



Shell of typical turrited Turbonilla shape. There is a considerable 

 range in size in the complex. The nuclear whorls are typically tur- 

 bonilloid; they form a helicoid apex whose axis is placed at right 

 angles to the axis of the postnuclear whorls. The postnuclear whorls 

 are crossed by axial ribs which vary in numbers, strength, and spacing 

 in different individuals. The intercostal spaces of the whorls show 

 an enormous range of differentiation in spiral sculpture in the com- 

 plex. While this sculpture is usually constant on all the whorls of 

 an individual shell, different individuals may have incised spiral lines 

 and pits varying in number from 9 to 31 in the material before us. 

 These spiral markings likewise vary in strength from mere lines to 

 broad pits, and their number of pits and incised lines varies greatly 

 in different individuals of the complex. The suture, base, and aperture 

 are typically turbonilloid. 



Following the customary procedure, I am designating U.S.N.M. 

 No. 561697 (pi. 10, fig. i) as type specimen. 



