70 



compressly expanded, posterior somewhat contracted; epidermis 

 generally horny, sometimes hairy ; hinge toothless, sometimes 

 slightly crenulated, with the ligament marginal and partially 

 internal. 



Two very characteristic groups of Mussels are included in this genus, 

 the Mt/tili proper, numbering upwards of fifty species, and the Dreissena, 

 of which there are some eight or ten. In the first, which are inhabitants 

 of the sea, ranging widely over both hemispheres, the lobes of the mantle 

 are freely open ; but in the last, which inhabit brackish and fresh water, 

 the mantle-lobes are united, except for the passage of the byssus-spiuning 

 foot, and where two siphonal openings are formed for inhaling water and 

 for ejection. In this peculiarity we have an indication of a character 

 which prevails under various modifications in all the lower acephala. The 

 simple orifice is first fringed, then tubed, until in My a and Panoprea it be- 

 comes a huge proboscis, formed of the tubes enclosed together in a sheath. 



The My till are characterized by an elongately fan -shaped growth, more 

 or less beaked towards the umboes, having the anterior side for the most 

 part compressly expanded and the posterior arcuately contracted. Two 

 species, M. ungulatus and Calif ornianus, attain a large size. M. tortus 

 and kirsutus are chiefly remarkable for the densely-fringed epidermis with 

 which the animal invests them. As examples of smooth, brilliantly coloured, 

 horny epidermis over a ground of pearly nacre, M. piclus and smaragdi- 

 nus may be quoted. All have a well-developed byssus spun from a groove 

 in the animal's foot.* 



The Mytili are used largely by fishermen for bait, and some amusing 

 statistical reports are given of the number of millions of individuals con- 

 sumed in this manner in the course of a season. The consumption of 

 mussels as an entremet or general article of food is more limited, many 

 having taken alarm at the stories that are told, and which, for aught we 

 know, may be true, of their unwholesomeness and deleterious qualities. 

 M. crassus, with a small Orepidula-like deck within the umbonal extremity 

 of the valve, has been selected for the type of a new genus, with the name 

 Sept if er. 



Species. 



1. Adamsianus, Dunk. 3. Africanus, Fan Ben. 5. atropurpureus, Dunk. 



2. Afer, Gmel. 4. Americanus, Reel. 6. angnstus, Phil. 



* "The byssus consists of a bundle of horny fibres or threads connected to the animal within 

 the shell on the one hand, and to the rock on the other. How this connection is effected was first 

 discovered and explained, in his usual copious and clear manner, by Reaumur. By placing mussels, 

 Mytilus edulis, in vases of sea-water, he found the following to be their manner of proceeding : 

 — Opening their valves the foot was first protruded, and, with various strains and stretches, gra- 



