86 AMERICAN JOURNAL 



branchs, this form of the branchiae sometimes occurs. The 

 term Tectibranch also conveys to the mind the same sense as 

 Scutibranch. The expression Tubnlibranch would seem to 

 indicate a tubalar form of the branchiae, when it only refers lo 

 the tubular form of the animal. In the case of the Heteropoda 

 the branchii-e are set aside, and a character of subordinate val- 

 ue, the form of the foot, is set on the same level as those of the 

 preceding divisions. The word Cyclobranch would perhaps be 

 the most suitable, but for the confusion between them and the 

 Inferobranchs, if taken literally. 



In fact, in the establishment of orders, the position and form 

 of the branchiffi, the form of the foot, and the general form of 

 the animal have all been placed in the same rank, without as- 

 signing to any one of them a marked pre-eminence over the 

 others. To remedy this defect the author proposes, taking the 

 respiratory apparatus as his basis, to select the most important 

 of its characters, and to establish the primary divisions upon 

 this. He considers the position of the branchiae to furnish the 

 most important character. The branchiae can only occupy 

 three positions : they are either. 



Completely external ; 



Or completely internal, and then concealed in a cavit}^ which 

 is itself covered by a shell which usually envelopes the animal; 



Or simply protected by an imperfect test, a condition inter- 

 mediate between the two preceding. 



Hence, after the separation of the Pulmonata as a distinct 

 subclass, we get three great divisions, — the Exobranchs, the 

 Stegibranchs, and the Endobranchs. 



I. The order of Exobranchs may be subdivided, according 

 to the point of the surface upon which the branchii^ are insert- 

 ed, into — 



1. JEji'hranchs, which have them on the back (i>or/5, Gl^ilelUna, 

 kc.) 



2. Perihranchs, which have them round the mantle {Triionia, 

 Olaucus, Scyllsea, Phcamocera, &c.) The Eolidse would be 

 allied to both the Epibranchs and Peribranchs. 



8. Hypohranchs (the Inferobranchs of Cuvier). The Tiietydes 

 would approximate all three Orders. 



4. Pleurobranchs, which have the branchiee on the side {Pleuro- 

 branchics, PJeurobranchidium, Laniogera, kc.) The Pleuro- 

 branchs lead both to the Stegibranchs by their small test, and 

 to the bulk of the Endobranchs by the pectinated form of the 

 branchiae. 



II. The order of Stegibranchs would include four di- 

 visions : — 



1. StegihrancJis proper, corresponding to the Tectibranchs of 



