30 Mr. W. Clark on the Lacunse. 



vertical fissure, within which the white ribbon-like spiny tongue 

 is seen at its alternate opening and closing; it is when extracted 

 nearly an inch long, and the anterior folded jaws are supported 

 by the usual corneous plates and buccal apparatus. The tenta- 

 cula are pale yellow-brown, each having two fine longitudinal 

 lead- colour lines running laterally from base to point, long, 

 setose, conically tapering to not a very fine point ; eyes small, 

 on short ofi'sets at the external bases. Foot at rest subcircular, 

 in action an elongated oval, well rounded in front and behind, 

 constricted moderately at the anterior third of the length, the 

 other two-thirds margined with light drab opake transverse 

 flakes, posteriorly puckered or jagged, with a central longitudinal 

 depressed line. The anterior part of the pedal disk forms a ter- 

 minal arcuated channel, which gives that part of the foot the 

 aspect of having a narrow upper lobe separated from the pedal 

 disk forming a pair of narrow labia ; the upper posterior portion 

 of the operculigerous membrane in this species is very slightly 

 laterally expanded, and though not emarginate with caudal ap- 

 pendages, as in Littorina pallidula, is often sinuated and some- 

 times finely cloven ; the operculum is nearly the shape of a ver- 

 tical section of a pear, brown horn-colour; nucleus sublateral, 

 leaning on the columella, with about 2^ gyrations, the last of 

 which rapidly coalesces with the outer margin of the plate. 

 The animal has the true alternate undulatory longitudinal 

 characteristic progression of the Littorince. The neck is sim- 

 ple, without lobes. The branchial plume is single on the left 

 side with 35-40 or more very fine long close-set pectinations, 

 many of which in certain positions of the animal may be seen 

 under the mantle without dissection. The orifice of the anus 

 and the canal of the sac of viscosity are seen on the right side ; 

 in the female the matrix and its orifice, with the anterior part 

 of the ovary, and in the male, the verge. The faecal pellets are 

 pale red and exactly oviform : I mention this point, as I have 

 sometimes found it a good distinctive aid. I have mentioned 

 rather more than can be seen without dissection ; and I will only 

 enter so far on the anatomy of this species, as to observe, that I 

 have carefully compared numerous specimens of the L. littorea, 

 L. pallidula and L. littoralis, and I can pronounce all the internal 

 organs, including the oesophageal ganglia, absolutely identical, 

 allowing for the modifications of colour in the different species. 

 The male organe generateur supplies the position of the matrix 

 in the female, and the testis that of the ovary ; the former springs 

 under the right tentaculum, and is composed of two poi'tions, 

 the basal one being a tumid, annulated or ridged figure, dentated 

 exteriorly, with the minute orifice at the external angle; the 



