242 CHITONIM;. 



found between the rectum and the posterior part of the 

 ovarian sac. 



It has been said that the body is subannulate : in a hundred 

 dissections we could not see much trace of such configuration, 

 or breaks in it to correspond with the segmental arrangement 

 of the valves ; only slight marks, the effect of pressure, were 

 observed. The connection of the Chitons with the Crustacea 

 is, I think, so very slight and remote as to require no further 

 notice. 



Having cursorily disposed of certain objections, we will 

 proceed to state our own views, and in their course, allude to 

 other objections and discrepancies. Though the Chitons are 

 in closer alliance with the Bivalves, anatomically, by the 

 arrangement of the circulatory apparatus, symmetry of the 

 branchiae, and by the absence of tentacula and eyes, than by 

 the external hard parts; still in them, there are points of 

 coherence which are not without their value : for instance, in 

 Pholas dactylus, which it almost immediately follows in our 

 method, though the bivalve portion is not broken into regular 

 segments, there are certain testaceous pieces, commonly, 

 though perhaps incorrectly, called accessories, in number six, 

 including the principal valves. We also find in the Chitons 

 a subsymmetrical division into eight segments of what I con- 

 sider essentially an integral patelloid cone, and as much 

 accessorial as those of Pholas; indeed both in one and the 

 other, these component parts are equally necessary and essen- 

 tial. I admit that no great stress ought to be laid on the 

 contrasted points; nevertheless, in conjunction with other 

 decided anatomical analogies, they have their weight in the 

 balance. 



Our view of the natural position of Chiton is after Dentalium, 

 with which it has marked affinities, and in immediate contact 

 with the Patelloid group, in which we regard, in almost every 

 respect, Fissurella as the point of comparison, as in it is seen 

 the same form of the cone, though entire instead of broken, 

 the same parity of the branchiae, a similar posterior anal de- 

 bouchure, and the attenuated mantle, gradually thickening, 

 in both genera, to a tumid coriaceous margin, which in Fis- 



