Mr. Robert Garner's Malncological Notes. 3G9 



of the termini of the intestine and excretory organs, which 

 circumstance we see to have evidently taken place in Fissu- 

 rella, Haliotis, Gadinia, &c. But no account is taken of the 

 effect of the transposition on the size &c. of the organs in the 

 above theory ; and, judging from the character of the torsion 

 which has taken place, we are disposed to think that it is the 

 right valve which preponderates, and not the left, althoufh 

 the tendency to the spiral form which we see in some bivalves, 

 as Isocardium, tallies best with the dextral direction of the 

 spire in ordinary Gastropods. Mr. Owen expresses himself 

 as adverse to this view, principally from the existence of the 

 operculum from the first in the embryo. In a few cases the 

 operculum, or clausium, may be a dismemberment of the 

 columella or, tantamount, of the posterior portion of a patelli- 

 form shell. Theoperculum oi Xavicella, for instance, is formed 

 by the posterior reflection of the afterpart of the shell-mantle • 

 and it answers to the shell of Crepidula, or the columellar 

 portion of the univalve CaJyptrcea ; in C. Dillicynii there is 

 the same part, but curved, and free at its margins, though 

 attached to the apex of the shell ; in C. rudis this has become 

 a complete cup, whence the name cup-and-saucer limpet -, whilst 

 in the pretty little C. sinensis, or Chinese bonnet, the lamina 

 goes to form the inner part and columella of a spiral shell. In 

 C. rudis it is puzzling to understand how the inner cup is 

 formed ; it perhaps answers to an open columella ; the forma- 

 tion of the other species is more intelligible. In ordinary cases 

 the operculum is formed on a distinct pallial lamina at the back 

 of the foot, occasionally connected above with the shell-pal- 

 lium, as in some Turbinidge. If we reject Oken's view alto- 

 gether, we are driven to suppose that the curiously modified 

 operculum is the homologue of the byssus of bivalves. In 

 Phasianella there is an equal development of both branchiae 

 separated by a septum, as in many Ceplialopoda ; and yet here 

 with little internal derangement, we have an operculum — this 

 being somewhat adverse to the above theories, tjiough it mifht 

 be accounted for in the modern mechanical way, the further 

 from the centre of rotation the greater the tendency to divide. 

 Two patelliform shells, then, very similar in form, may 

 belong to animals of quite different families : one may cover 

 a dioecious and carnivorous mollusk with only a single bran- 

 chial process, and a heart with simple auricle and ventricle, 

 with the intestine opening on the right side ( Calyptrcm, Cre- 

 pidula,nn({IIipj)onyx) ; another a monoecious and phytophagous 

 animal, with two symmetrical branchial organs, and' heart with 

 double auricles and perforated ventricle (Fissurella). In the 

 first case one of the branchial laminae and the corresponding 



