BIVALVES— ARA'S, NUT-SHELLS, ETC. 621 



silium borne in a small triangular pit in the middle. The umbonea are 

 directed backwards. Their close allies, Lcda and Yoldia, are elongate, and 

 have fewer and more prominent teeth, and a more prominent ligament pit. 

 The family is one of the oldest known, dating back to the earliest geological 

 times, and its modern representatives are found in all seas. 



The SoLENOMTli).^ comprise a single genus Solenomya, and are remarkable 

 for being without teeth in the hinge, and in having the perioatracum greatly 

 prolonged beyond the margin of the valves. The typical species, S. togata, 

 is found in the Mediterranean. The genus also occurs on the east coast of 

 North America, the Antilles, Patagonia, New Zealand, and Australia. 



OrDEK II. — FlLIBEANCHIATA. 



In this order the gill filaments are dependent and reflexed with ciliar, but 

 no interlamellar junctions. The animal generally spina a strong byssus. 

 Four sub-ordera are distinguished : — • 



Sub-order 1. — Anomiace,^?, whose typical genus Anomia is well-known by 

 the common A. ephippium, found on our shores. The shell is very variable 

 in shape, but the right valve ia generally the flatter of the two, since it is 

 moulded to the surface of the object the animal is attached to. The strong 

 byasus, by which it holds on consists of a shelly " plug," instead of a bundle 

 of threads, and passes through a notch in the right valve near the umbo. 

 AVhen quite young the notch is in the ventral margin of the valves, as in the 

 next group, but the shell subaequently grows round it. There are no teeth 

 in the hinge ; only a aingle adductor muscle, the posterior one, is present in 

 the adult; and the heart projecting into the pallial cavity lies almost with- 

 out the body. The Saddle oyster, Flacuna sella, also belongs to this group. 



Sub-order 2. — Arcaoea, of which the Noah's Ark- 

 shells are the type (Fig. 8), have hinge-teeth like JVwciiZa; 

 but the ligament ia external, extendaforwardinadvance 

 of the umbones, and spreads over the wide space be- 

 tween the two beaks, frequently obtaining additional 

 hold by grooves in the shell. In Area there is often 

 a strong and sometimes shelly byssus, which passes 

 through an opening in the ventral margin of the shell. 

 The two adductors are nearly equal in size. The 

 mantle margins are entirely open. The genera Pec- Fi(j. 8. 



tnnculus and Limopsis have much rounder shells ; the *. Arca Nc^e.^ 



teeth are far fewer, and less comb-like than in Area. 



All three genera are represented on our coasts, and have a wide distribu- 

 tion ; whilst Area dates back to almost the earliest geological times. 



Sub-order 3.— Trigoniacea, represented by a aingle family, TRiooNiiDiE, 

 having one genus Trigonia. This genus, now confined to the Australian 

 seas, was in geological times very widely distributed. The living species are 

 beautiful pearly sheila, roughly triangular in shape and externally ribbed, 

 after the manner of a cockle-shell. The right valve benrs two cardinal teeth, 

 the left four. These teeth are transversely grooved. There is no byssus. 



Sub-order 4.— Mytilacea is so well known through its representatives the 

 common marine mussel, Mytilus edulis, and the Horse-mussel, Modiola 



