MOLLUSCA. 
61 
shores being absent. A worn valve of Isocardia cor found on 
the West Sands is purely accidental. Tellina pusilla and 
Psammobia tellinella are uncommon at St. Andrews. Amongst 
univalves, Trichotropis borealis, Pleurotoma Trevelyana, 
Aplysia punctata, and Philine pruinosa are noteworthy. 
The smaller univalves, such as Rissoce and their allies, are 
much less numerous in species than on the southern and west¬ 
ern shores, the absence of Trochus umbilicatus being especially 
diagnostic when contrasted with the latter. The Nudibranchs 
are well represented at all seasons ; and the individuals in the 
majority of the species are numerous. Ommatostrephes and 
Loligo amongst the cuttles often occur in great profusion on 
the West Sands after storms. 
On the whole the species are northern, and stand in strong 
contrast to the molluscan fauna of the western shores, where 
Thracia convexa, Tapes decussatus, Pecten varius, var. nivea, 
Teredo megotara and T. norvegica, Fissurella, Trochus umbili¬ 
catus and T. zizyphinus in the littoral zone, and the abund¬ 
ance of T. magus and T. tumidus in the laminarian, Phasianella , 
Ahera bullata, Elysia, swarms of large and small Rissoce, and 
the pelagic Ianthina form conspicuous features of the marine 
fauna, just as the hosts of Bulimus acutus do on the sandy 
fields of Killipheder and other parts of the extreme west. 
Still more evident is the contrast with the rich southern species 
that cluster round the Channel Islands—such as the finely 
developed pectens, oysters, and Anomice , and the appearance of 
the former between tide-marks (P. varius ), besides Mytilus 
barbatus (which takes the place of the bearded varieties of our 
Mytilus modiolus) in obscure crevices in the littoral zone, the 
frequency of Area tetragona in fissures of the rocks, Galeomma 
on the under surface of stones in tide-pools at Herm, the boring 
Gastrochcena in shells, and the abundance of Ilaliotis, Pandora, 
Venus verrucosa and V. ovata , Mactra glauca, the Psammobiie , 
and the “angel’s wings” {Lima), which when disturbed flit 
with their brilliant orange fringes so nimbly in the tide-pools. 
Amongst univalves, again, the large size and abundance of 
Chiton discrepctns, Fissurella, Emarginula, Murex erinaceus, 
Aplysia punctata, Eulima polita, Trochus lineatus, Cerithium 
and Ceritliiopsis, and the predatory and cunning cuttles 
