GASTEROPODES. 285 



SECTION II. TRITONIA. Body fleshy, solid, lanceolate. Head ham- 

 mer-shaped. Tentacula two, simple, retractile, in long trumpet sockets. 

 Branchiae marginal, erect, tufted, or composed of successive stages. 



1. TBITONIA PINNATIFIDA The Crested Doris The Crested Tritonia. 

 Plate XLI. Figs. 1, 2, 3, 4, 5. 



The transitions of form of the different parts of these animals render 

 it difficult to comprehend more than very few under the same paragraph. 

 Both the species of the preceding section are distinguished by the pecu- 

 liar form and arrangement of the tentacula, branchiae, and coils of the 

 spawn. But the head is scarcely to be called of any definite form as such. 



Here the head is more definite, being somewhat in the form of an 

 ordinary hammer. As we advance in considering the species of the tribe, 

 we shall find successive changes in this part of the animal, such as may 

 lead to the foundation of new divisions. 



Length of the Tritonia pinnatifida an inch, breadth two lines. Body 

 lanceolate, shoulder rounded. Head hammer-shaped, having a projection 

 on each side. Tentacula two, rising upright from sockets, susceptible of 

 considerable prolongation. A row of marginal branchial tufts on each 

 side of the back, which is bare and the skin smooth. Colour white, 

 dingy brown, yellowish, or greenish.* 



The figure of the head and tentacula are constant features : the 

 number of branchial tufts, their dimensions, and the colour of specimens, 

 are variable. 



One specimen had nine pair of branchiae, another eight, which latter 

 number, if I mistake not, Dr Johnston ascribes to the animal, in his valu- 

 able contributions to the Scotish Fauna, observing, at the same time, 

 that various authors, whom he quotes, derive their descriptions from 

 Montagu. I had a third specimen with seven pair, some with still fewer. 



* Montagu, in Linnsean Transactions, vol. vii. p. 78, Plate VII. fig. 2. Although 

 this figure is not satisfactory, it certainly indicates the species mentioned here. 



