424 MOLLUSCA SUB-KINGDOM VI 
changes the appearance of the shell that old and young stages have frequently been 
described as specifically or even generically distinet. 
Typical Pholads date from the Jura. Many sub- 
genera have been named. 
Turnus, Gabb (Fig. 785). Cretaceous. Martesia, 
Leach (Fig. 786). Carboniferous to Recent. Jowan- 
netia, Desm. Tertiary and Recent. Teredina, Lam. 
Valves in the adult stage soldered together and to a 

Fic. 785. thick adventive caleareous tube. Eocene. 
Turnus (Xylopha- one aS 
gella) elegantulus, Fic. 786. : 
Meek. Upper Cre- Martesia conoidea, Family 46. Teredinidae. Scacchi 
taceous; Idaho. Desh. Eocene; < i : 
Enlarged (after Auvers, near Paris. $ 5 4 ; 
Meek), 1/,. Shell much reduced, equivalve, auriculate, widely 
gaping, the valves apposited ventrally only on the 
surface of a parietal tubercle; adductor scars wnequal, the anterior marginal very 
small ; pallial line coincident with the valve margins: a 
styloid myophore projecting from the cavity of the beaks ; ~ B Cc 
mantle secreting a caleareous lining to the burrow ; pallets ; 
variable in form, the valves without attached accessory shelly 
plates; area none; hinge margin reflected, edentulous : 
ligament absent or obsolete; anterior adductor degenerate, 
attached on the anterior edges of the valves, and covered only 
by the mantle; animal boring, chiefly in wood. Carbon- 
iferous (7), Jura to Recent. 
Teredo, Linn. (Fig. 787, A, (). Pallets simple, spatuli- 
form. Jura to Recent. 
Xylotrya, Leach (Fig. 787, B). Pallets articulated, 
bipinnate. Tertiary and Recent. 
The name Teredolites, Leymerie, has been proposed for 
the casts of borings of fossil Teredos (Fig. 787, D). The 
problematical genus Polorthus, Gabb, from the American 
Cretaceous, has been referred to this family. The Palaeo- 
zoie species are known only by burrows, which are of 
somewhat doubtful origin. 

Fic. 787. 
A, Valves of the recent Teredo 
Norvegica, Spengl ; innerand outer 
views. B, Pallet of Xylotrya sp. 
Vertical Range of the Pelecypoda. C, Pallet of Teredo sp. D, Casts 
of borings of Teredo Tournali, 
Pelecypods make their appearance as extreme rarities ate ROE ee 
in the Lower Cambrian,! being represented only by the 
doubtful Fordilla and the tiny Modioloides. Even in the Ordovician they are still 
rare. Modiolopsis, Ctenodonta, Palearca, Eopteria, and Glyptarca have been sparingly 
found. In the Silurian a considerable augmentation of the number of bivalves is 
observable, as many as eighty species having been distinguished in the fauna of the 
small island of Gottland alone. 
A very marked difference in geological range is perceptible among the three orders 
into which the class is divided. The Prionodesmacea, including most of Neumayr’s 
Palaeoconcha, are pre-eminently characteristic of the Palaeozoic faunas. Of the forty- 
two families referred to this order, no less than nineteen occur in the Silurian, to which 
seven are added during the Devonian, only three in the Carboniferous, and one in the 
Permian. From these ancient stocks only seven Prionodesmacean families are evolved 
during the whole of the Mesozoic, and but two in the Tertiary, while three are Recent. 
Omitting offshoots, but four families cover the whole range of geologic time from 
1 Cf, Walcott, C. D., Fauna of the Olenellus Zone (10th Ann. Rep. U. 8. Geol. Sury., 1890). 
