Angust-Scpt., 1914. 
The Irish Naturalist, 
161 
THE OPISTHOBRANCH FAUNA OF THE SHORES 
AND SHALLOW WATERS OF 
COUNTY DUBLIN. 
BY NATHANIEL COLGAN, M.R.I.A. 
The present paper is based chiefly on the results of 
numerous dredgings and shore gatherings made on the 
Co. Dubhn coasts within the past nine years during which 
special attention was paid to the Opisthobranch group of 
marine mollusca. The most important section of this group, 
the Nudibranchs, had been but little studied on the Dublin 
coasts when the systematic exploration of their mollusca 
was taken in hands by the writer towards the close of 1905. 
At that time only 26 species of Nudibranchs proper, were 
on record for the county fauna, not a single member of the 
Ascoglossa, now usually classed with the Nudibranchs, was 
known to inhabit the area, and the total of Opisthobranch 
species for Dublin waters stood at 43. To-day the Opistho- 
branch fauna of the county numbers 60 species, including 
45 Nudibranchs proper, and 4 Ascoglossans ; and there is 
reason to believe that further exploration would add several 
species of this most interesting and beautiful group to the 
already rich fauna of the Dublin coasts. 
The seaward boundary of the marine area dealt with in 
the following pages has been fixed at the conventional 
limits of what are known as territorial waters, or roughlv 
at a distance of 3 miles from low water mark of the Dublin 
shores, and the inshore waters of Lambay, an island lying 
2\ miles distant from the nearest point of the mainland, 
have been included in the county limits. This area is a 
distinctly shallow water one. In some points, as off Howth 
Head, off Dalkey Island and off the Nose of Lambay, sound - 
• ings up to 18 fathoms may be had ; in many places and 
over considerable tracts the soundings range from 10 to 15 
fathoms ; but nowhere does the depth attain to 20 fathoms, 
and the greater part of the area gives soundings ranging 
from 3 to 8 fathoms only. The form of the shores and of 
the sea bottom is sufficiently varied. At Howth Head, at 
A 
