NOTES ON THE RADULA OF THE HELICINIDAE. 



By H. Burrington Baker. 



This paper is the result of an attempt to ascertain the generic 

 position of three species of Helicinidae, which were collected by 

 the University of Michigan- Walker Expedition in Southern Vera 

 Cruz, Mexico (1910). This led to the examination of the type 

 species of characteristic examples of many of the groups of North 

 American Helicinidae. The majority of the radulae was studied 

 from alcoholic or dried material in the collection of The Academy 

 of Natural Sciences of Philadelphia, kindly put at my disposal by 

 Dr. H. A. Pilsbry, whose advice and criticism were of invaluable 

 assistance. I am indebted to Mr. E. G. Vanatta and Dr. C. 

 M. Cooke for help, especially in technique. Mr. John B. Henderson 

 also sent me several Cuban species from his own collection. 



Three of the species were studied from the University of Michigan 

 collection from Mexico, to which reference has just been made, 

 while additional material came from a collection made in Venezuela 

 in 1920. As the study was attempted in order to arrange Mexican 

 forms, a synopsis of the North American mainland species is 

 included in the notes on each group. I hope to take up the 

 species from northern South America in a future paper. 



A. J. Wagner (1905, 1907-1911) made the most thorough 

 attempt, in recent times, to arrange the species of Helicinidae. 

 His classification is based solely on shell-characters, mainly the 

 shape of the operculum and the position of its nucleus. Although 

 a great advance on earlier attempts, his work is seriously marred 

 by a general disregard of previous writers on the subject, and of 

 the modern laws of priority. Of the 36 new group-names, proposed 

 by him for American Helicinidae, only about 16 can be used. 



The embryonic operculum of the Helicinidae is spiral and its 

 spiral nucleus is evident in the adults. Thus the adult operculum 

 may perhaps be regarded as potentially spiral, with the rotation 

 arrested by the shape of the aperture. The microscopic structure 

 of the columellar margin of many species certainly adds to this 

 impression, as the growth-lamellae are crushed and bent outwards 

 in this region, as if they actually had resisted an attempted torsion. 

 The position of this thickened region varies with the shape of the 

 aperture. 



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