420 Zoological Sociefi/ . — 



fifteen years since I first observed in the marshes near the bank of 

 the Thames, between Greenwich and AVoolwich, in company with 

 Bj)ecies of J'dfrnfa, liitJnjnid and PisuHinii, a small nnivalve shell, 

 agreeing with the smaller s])eeies of the littoral genns Littorina in 

 every character both of shell and operculnm. Yet this very pecn- 

 liar and, aj)pareiitly, local species has an animal which at once distin- 

 guishes it from the animal of that genns and from all Ctenobranchons 

 Alollusca. Its tentacnla are very short and thick, and have the eyes 

 placed at their tij>s, while the Littor'uicv, and all the other animals 

 of the order to whieh they belong, have their eyes placed on small 

 tidiercles on the outer side of the base of the tentacles, which are 

 generally more or less elongated. The shell in question and its 

 animal were described and figured by Dr. Leach in his hitherto un- 

 published work on British SloUusca, under the name of Assimmia 

 Grayana, and as this name has been referred to by Mr. Jeffreys and 

 other conchologists, it may be regarded as established, and that of 

 Si/ncera hepatica, ])roposed by myself in the ' Medical Repository,' 

 vol. X. p. 230, will take rank as a synonym. A second species of 

 this genus has lately been niade known by Mr. Benson, by whom it 

 was found on the ponds in India. Its shell is banded like that of 

 Littorina \-fasciata and several other smaWer Littorince, and has 

 been figured in the Supplement to ' NA^ood's Conchology,' t. 6. f. 28, 

 under the name of Ti'i-Ijo Francesice." 



In my edition of 'Tnrton's Manual,' 1840, p. 88, I characterize 

 the genus thus : — Assiniinia : Shell ovate, conical, solid ; mouth 

 ovate ; tentacles very short, scarcely longer than the tubercles on 

 which the eyes are placed, and united to their side, p. 78, f. 1, 5, fi, 

 observing, " the animal differs from Littorina in the apparent posi- 

 tion of the eyes, which is an anomaly among the water and Cteno- 

 branchous Mollusca ;" and after quoting Mr. Berkeley's description 

 of the tentacnla I observe, — " I am inclined to retain my former 

 theory, for if the pedicel of the eye of this genus is minutely exa- 

 mined, it will appear to be formed of two parts united by a suture." 



In 1852, having obtained permission of the family, I printed 

 Dr. Leach's ' Molluscorum Britannise Synopsis ' above referred to, 

 and he there described the genus — " Assiminia. Testa conica, 

 spira mediocris. Animal tentacnlis duobus brevibus, apice paulo 

 angustioribus obtusis, ad apicem oculigeris, instructum ; oculi parvi, 

 rotundi ; operculum tenue." 



" From the form of the shell this genus might be considered as 

 belonging to the second stirps {testa conica, spira hrevis), but the 

 animal proves that it is more nearly allied to Sahancea than to any 

 other of the British genera." (p. 1.05. t. 9. f. 4, 5.) 



Lately some doubt has been attempted to be thrown on the 

 distinctness of the genus, which it has been proposed should be 

 united to the genus Trvncatella of Risso. 



Considering the very great similarity which often exists in the 

 general appearance of the animals of very distinct genera of Mollusca, 

 — a similarity so great, that if a person was to place before me, without 

 the shell or operculum, the animal of the genera iV/j/rear, Triton, Pur- 



