SCAPHOPODA. IX 



eapillosum (pi. 8, fig. 34) this condition occurs either nor- 

 mally or in exceptional cases. All of them agree in other 

 characters with the subgenus Fissidentalium, and are herein 

 referred to that group. 



VIII. Two symmetrical lateral slits (pi. 27, figs. 90-92). 

 IX. Four or more slits cutting the apical margin into lobes 

 (plates 28, 29, 30). 



These characters have been considered sufficient for the definition 

 of genera by Stoliczka and some other authors. Dall, on the other 

 hand, (Trans. Wagner Inst., iii, 436), attaches no systematic import- 

 ance to the various modifications, which he attributes largely to 

 erosion and repair of breakage. Neither of these positions seem to 

 us tenable in the radical sense in which they have been advanced. 

 The apical characters are subject to much variation in many species, 

 but they still have considerable value as specific and group char- 

 acters. It is no valid argument against the systematic value of the 

 apical teeth in (for instance) Polyschides, to show that they are fre- 

 quently broken off, although that argument might be held conclu- 

 sive against using the character as absolutely diagnostic in the de- 

 termination of species or genera. Again, the accessory tube, de- 

 scribed in paragraph II above, is not due to repair of an accidental 

 breakage, but is a normal process following absorption of the shell, 

 and occurring only in the species of certain subgenera of Dentalium, 

 although not developed in every specimen of these species, and oc- 

 casionally in but a small proportion of them. The forms possessing 

 this structure are not " especially liable to such breakages " more 

 than others which never develop the added tube. The theory that 

 " from a peculiar fragility or liability to transverse breakage in a 

 species, this condition may [become] almost habitual with the adults 

 of that species" is not supported by any evidence we have encount- 

 ered ; and the supporting statement that " no one has ever recorded 

 a specimen with the posterior end entirely unbroken and yet pos- 

 sessing the supplementary tubule," loses its weight when it is re- 

 membered that no Scaphopod can in the nature of things retain 

 41 the posterior end entirely unbroken " beyond the earliest stages of 

 growth, the absorption of this end being as essential a process as 

 growth at the other. 



While we do not consider the characters of the posterior orifice as 

 so unreliable as some authors have thought, it must not be gathered 

 that we place great weight upon them. That there is a wide range 

 of variation among individuals of the same species is sufficiently 



