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I4O7 



TERTIARY FAUNA OF FLORIDA h ' 



even absent, but the trace which exists is usually circumscribed by a deep sulcus, 

 the termination of which on the left inner hinge-margin is usually marked by 

 a little pustule. I was at first disposed to associate this prominence with the 

 anterior left lateral, which often is superimposed upon it, but finding in several 

 species both lateral and pustule present and not coincident, I concluded the 

 latter simply represents the dynamic effect of the structure of the tissues which 

 secrete the lunular margin. 



The sulcus in Beguina has been so affected by the torsion-like manner in 

 which the hinge has been drawn out and twisted that it actually has become 

 tubular, though in life probably closed by an organic plug of some kind. 



The escutcheon is frequently present, not bounded by a sulcus or other 

 emphatic limit, but rather indicated by a more or less distinct carina. 



In general the members of this family may be divided into elongate species 

 which are usually byssiferous, and short or subtriangular forms in which there 

 is no gape or byssus in the adult shell. 



We know nothing of the characteristics of many of the recent animals of 

 this group, but considering those of which we have information it appears that 

 the species are dioecious, and the females have a tendency to retain the young 

 until a start has been made in growth beyond the prodissoconch stage. In the 

 triangular forms, such as Venericardia borealis Conrad, the young in large 

 numbers are incubated in the ovary or its atrium in the umbonal cavities of 

 the valves. In some of the mytiloid species a marsupium is formed by the 

 ventral margins of the mantle (Thecalia, Milneria), which secrete and line a 

 shelly pouch on the ventral side of the valves in which the eggs are hatched, 

 and the nepionic shells are for a time retained. 



The prodissoconch is usually smooth and ovate, sometimes with the extreme 

 margins thickened and projecting, while the nepionic sculpture of radial ribs 

 commences suddenly. 



The sculpture in this group is predominantly radial, usually strong ; Calyp- 

 togena alone has no radial sculpture. The pallial line is almost invariably simple 

 and entire ; a broad scar, which may be connected with the siphonal sphincters, 

 occurs in Cardiocardita ajar. The ligament and resilium are external. 



The species seem to affect rocky or gravelly regions from low water to 

 moderate depths, rarely exceeding one hundred fathoms except in a few cases, 

 where shallow-water Arctic or boreal species follow the cold water southward 

 into gradually increasing depths. The distribution is world-wide, the mytiloid 

 forms being, however, restricted to the warmer seas. 



The species of this family may be arranged tinder the following groups : 



