240 LIFE BY THE SEASHOEE. 



further than to re-emphasise the peculiar flatness of the 

 spire. 



The four types given here have been chosen because I 

 have found them to be the most abundant on the shores of 

 the Firth of Forth, but the periwinkles of any area form a 

 most interesting study. 



Related to the periwinkles are two genera of minute 

 shells, which we can only mention without description. 

 The first of these is the genus Rissoa, which includes a 

 great number of British species, inhabiting very various 

 depths of water. To obtain examples pluck a good handful 

 of any of the finer seaweeds, and drop into a dish of sea- 

 water. Presently there will crowd to the surface numerous 

 minute forms with spirally coiled shells often beautifully 

 sculptured. They are active little creatures, crawling over 

 the seaweed, or taking advantage of surface tension to creep 

 along the surface of the water, shell downwards. The 

 other genus is Skenea, including especially Skenea planorbis, 

 a common shore form with a circular depressed shell. It is 

 very minute, being just visible to the naked eye. 



On many parts of the coast " tower-shells " (Turritella) 

 are found very commonly thrown on the beach. There is 

 only one British species (T. communis), and it is an 

 inhabitant of deep water, so that the living animal is not 

 likely to be found. The shell is elongated and tapering, it 

 has sometimes as many as nineteen whorls, of which the 

 first ten bear three distinct ridges. The aperture is entire 

 and rounded. The shell is usually of a brownish colour, 

 and may be over two inches in length. 



Though Molluscs which only occur in the dead state are, 

 strictly speaking, somewhat outside our scope, we must 

 mention the curious " pelican's foot," Aporrhais pes-pelecani, 

 which is not infrequent on the shore. The shell is turreted, 

 very strong, with numerous ornamented and ribbed whorls. 

 The mouth of the shell is furnished with a short canal, 

 and in the adult its outer edge is expanded into a large 

 lobed plate. The shell is interesting on several accounts, 

 especially because it is in some respects transitional between 

 the Azygobranchs, with round entire aperture like Trochus, 

 and those with canaliculate, or notched aperture like 

 Bvacinum. The living animal is beautifully flecked with 



