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 distinct. 

 Typical Pholads date from the Jura. Many sub- 

 genera have been named. 



Tnrnus, Gabb (Fig. 785). Cretaceous. Martesia, 

 Leach (Fig. 786). Carboniferous to Recent. Jouan- 



\ ' iiLMF *vOi ^i^P net ' DeSm ' Tertiar y and Decent. Teredina, Lam. 

 l VI WM Valves in the adult stage soldered together and to ;i 



thick adveiitive calcareous tube. Eocene. 

 Turntis (Xylapha- 



f/ella) elegantuhts, 

 Meek. Upper Cre- 

 u s : Ida 



t a c e o u s ; Idaho. 

 Enlarged (after 

 Meek). 



Fi. 786. 



Murtcsia conoidc.a, 

 Desh. Eocene; 

 Auvers, near Paris. 



Vi- 



Family 46. Teredinidae. Scacchi. 



Shell much reduced, equivalve, auriculate, widely 

 gaping, the valves apposited ventrally only on the 

 surface of a parietal tubercle: adductor scars unequal, the anterior marginal very 

 small; pallial line coincident ivith the valve margins: a 

 styloid myophore projecting from the cavity of the beaks : 

 mantle secreting a calcareous 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 (?), Jura to Recent. 



Teredo, Linn. (Fig. 787, A, C}. 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- 

 zoic species are known only by burrows, which are of 

 somewhat doubtful origin. 



Vertical Range of the Pelecypoda. 



A, Valves of the recent Temlo 

 Norveffica, Spengl ; inner and outer 

 views. Ji, Pallet of Xylot.rya s]>. 

 C, Pallet of Teredo -sp. D, Casts 

 of borings of Teredo TnnninH, 

 Leym. Eocene ; Kressenberg, 

 Bavaria. 



Pelecypods make their appearance as extreme rarities 

 in the Lower Cambrian, 1 being represented only by the 



doubtful Ford-ilia 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, 110 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 Prioiiodesmacean 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. Wahott, C. />., Fauna of the Olenellus Zone (10th Ann. Rep. U. S. Geol. Surv., 1890). 



