MOLLUSCS, OB SHELL-FISH. 233 



they offer the same contrast as Patella and Helcion, for 



while the tortoise-shell limpet is to be found not far below 



high-tide mark, Acmcea virginea is only to be found among 



the LaminarioB. We have, however, already (p. 24) seen 



that Acmcea testudinalis differs from the common limpet in 



being confined to the pools, and in never 



climbing high above the water level, as 



Patella vulgata does. It is a pretty little 



creature, easily recognised by the distinct 



pattern in brown on the shell, and the very 



dark brown " spatula " or mark in the inside 



of the shell. The mantle is bright green FIG. 68. Tortoise- 



and the eggs bright red, so the animal does (AcmLa l lestudt 



not lack bright pigments. The allied form, naiis). 



A. virginea, is of similar size and appearance, 



but has the shell ornamented with rays of pink instead of 



the brown network of the other species. The spatula is 



not brown, and the mantle has not the vivid green colour 



of the tortoise-shell limpet. It will be found attached to 



shells and stones in the Laminarian zone. On the South 



and West Coasts other limpets will be found in addition to 



these four. 



After the limpets we come to the Azygobranchs peri- 

 winkles, whelks, dog-whelks, and similar forms, with strong 

 spiral shells and active habits. To make sure of them 

 collect on the shore a handful of common forms, such as 

 the large whelks, very common in some places under muddy 

 stones, a few periwinkles, some living tops ("silver Tommies"), 

 or any other spirally coiled shells which catch your eye, and 

 drop them into a basin of clean water. Note the different 

 shapes of the shells, largely dependent upon the number 

 and shape of the coils, and also the fact that while some of 

 the shells have smooth rounded mouths, others, such as the 

 whelks, have the mouth prolonged into a canal. Note, too, 

 the general characters of a coiled shell; all have a centra] 

 pillar or columella, a large body-whorl, a spire made of other 

 smaller whorls, a mouth with outer and inner lip, and so on. 

 Another point will be very obvious, namely, that not only 

 can the animal retreat into its shell in a way which is 

 impossible for a limpet, but that when it does retreat it 

 " shuts the door " behind it, by means of a firm plate, or 



