X SCAPHOPODA. 



shown in the descriptive portion of this work. That apical charac- 

 ters usually cannot be held sufficient for generic and subgeneric 

 distinctions is recognized by our use of other features, especially 

 sculpture, for this purpose. And on the whole, while we cannot en- 

 dorse all the arguments advanced by Dall to support the position, 

 we thoroughly agree with his conclusion " abnormalities may usu- 

 ally be discriminated by comparison with numerous specimens of 

 the same species. In cases where the student has only one or two 

 specimens, he should refrain from putting reliance on characters 

 which may be abnormal as a basis for describing new forms or for 

 discriminating old ones." 



In many species, especially the groups of D. entalis and D. semi- 

 striatum, the young shell is sculptured while the later growth is- 

 smooth. Frequently the adult retains some of the sculptured por- 

 tion posteriorly ; but in some individuals or species this early sculp- 

 ture is entirely lost by posterior truncation. Such forms are prac- 

 tically indistinguishable from species which are without sculpture at 

 all stages of growth, although belonging to quite different groups. 

 Young or half-grown specimens show the true relationships in these 

 cases. 



SPURIOUS SCAPHOPODS. 



Throughout the early period, various Gastropod mollusks such as 

 Caecum were occasionally described as Dentalium ; and until Berke- 

 ley defined the genus Ditrupa in 1834, demonstrating its Vermian 

 nature, the calcareous tubes of species of this genus and of Pomato- 

 ceras were commonly referred to Dentalium. Palaeontologists, how- 

 ever, almost up to the present decade, have described the tubes of 

 worms of the family Serpulidce as Scaphopods. So general has been 

 this error, and so widely is it spread throughout the literature of 

 Scaphopoda, that we have considered the pseudo dentalia in a sepa- 

 rate section (page 240) of this volume. 



HABITS, FOOD, AND USE BY MAN. 



According to Lacaze-Duthiers, the Mediterranean Dentalium pre- 

 fers to live in clean and rather coarse sand, and avoids mud con- 

 taining decomposition products. Many deep sea forms live in mud. 



The animal lives buried at an angle of 45 or less with the sur- 

 face, the posterior end only projecting. Their food consists of 

 foraminifera, minute bivalves, and, it is said, infusoria. The prin- 

 cipal enemies of Scaphopods seem to be molluscan. They have been 

 found in the stomachs of Scaphander and other opisthobranchs, and 



