THE present volume is devoted to the monographs of Tecti- 

 branchiate mollusks, in continuation of Volume XV. The 

 CEPHALASPIDEA herein monographed belong to groups of 

 which but few species are represented in'the collection studied, so 

 that little beyond the merits of a careful compilation can be ex- 

 pected. In the ANASPIDEA, more material has been available, and 

 it is hoped that progress has been made in the classification and defi- 

 nition of subfamilies and genera, as well as in the facilitation of 

 specific determinations. To a less extent this also applies to the 



NOTASPIDEA. :.\i^ . 



A single family of ASCOGLOSSA, Oxynoeidce, is included herein, on 

 account of the Bulla-like shell developed. The other ASCOGLOSSA 

 and the NUDIBRANCHIATA proper, which have no shells when adult, 

 will not be included ; the fruitful labors of Bergh rendering their 

 treatment here inadvisable, especially in view of the fact that few 

 conchologists concern themselves with those groups. 



In an appendix, the Tectibrauch groups of Volume XV are 

 brought up to 1896. 



A brief introductory chapter embodies the views of the author on 

 the classification of CEPHALASPIDEA ; the chief departures from 

 previous arrangements being in the dismemberment of the old family 

 Bullidce, with the creation of Akeratidce; and the rearrangement of 

 the families into phyla based largely upon the mode of specialization 

 of the radula, and the development of the pleuropodial lobes. 



H. A. P. 



in 



