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 decussaius, 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 laminar ian, 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 Hal iot is, Pandora, 

 Venus verrucosa and V. ovata, Mactra glauca, the Psammobice, 

 and the " angel's wings " {Lima), which when disturbed flit 

 with their brilliant orange fringes so nimbly in the tide-pools. 

 Amono-st univalves, again, the large size and abundance of 

 Chiton discrepans, Fissurella, Emarginula, Murex erinaceus, 

 Aplysia punctata, Eulima polita, Trochus lineatus, Centhium 

 and Ccrithiopsis, and the predatory and cunning cuttles 



