3S Mr. Thompson on the Mollusea of Ireland. 



marked that the appearance tU'scribcd as an orifice behind 

 the vent, " is merely formed by the partial apposition of the 

 edges of a slit existing in the posterior margin of the mantle, 

 and which approximation is dependent on the will of the 

 animal. M In addition to these specimens, beautifully coloured 

 figures, both of the natural size and magnified, drawn by Miss 

 Allman from the living mollusk, illustrate the above points. 



DortS don (jut a, mihi. PI. II. fig. J. 



(roniodoris* do. do. 



D. elongated, narrow, a row of papillae on each side the back, 

 branchial filaments about 10, plumose. 



Length of specimen (from spirits) 3 lines, breadth 1 line, height 

 J line, breadth of body equal throughout. 



This species resembles in form the D. gracilis and D. pal- 

 lens of Rapp. Nova Acta, vol. xiii. part 2. p. 522. tab. 27. 

 fVs. 9 and 10. 



I obtained this mollusk in June 1838, between tide-marks, 

 at the island of Lambay, off the Dublin coast. 



Tritonia Hombergii, Cuv. Johnst. Ann. Nat. Hist. vol. i. 

 p. 114. pi. 3. figs. 1 and 2. 



A specimen about 4 inches in length, with the examination 

 of which I have been favoured by Mr. R. Ball, w r as dredged 

 with oysters some years ago at Howth, county Dublin. 



Tritonia lactea, mihi. PL II. fig. 3. 



T. of a milk-white colour, with 6 large branchial appendages on 

 each side, bifid and ramosely pinnate ; mantle terminating anteriorly 

 in 4 arborescent processes. 



Length of specimen (from spirits) 8 lines, sheaths of the tenta- 

 cula deeply fimbriated. Colour milk-white, but with the aid of a 

 lens a few very minute scarlet dots are seen scattered over parts of 

 the body and the branchial appendages. 



The approximation of this species to the T. arborescens, 

 Cuv. renders necessary some notice of the characters in 

 which they correspond and differ from each other. The dia- 



* A new and well-marked genus constituted by Mr. Forbes. See present 

 No. of Annals, p. 104. 



