NO. 1139. DEEP-WATER MOLLUSCAVEEEILL AND BUSII. 849 



simple "foliobranchiate" (or protobrancliiate) gills; two pairs of large 

 labial palpi, the outer ones furnished with long extensile labial tenta- 

 cles; a large muscular foot with an expanded, concave, terminal disk, 

 adapted for rapid motions in jumping and swimming, as well as for 

 creeping; and all have two series of transverse or oblique teeth on 

 the hinge-margin. The peculiar structures of foot and gills appear 

 together elsewhere only in the family Solemyida-, which is evidently a 

 related group, though it lacks hinge-teeth and has a very different shell. 

 As these three families have gills of a peculiar and simple structure, 

 each one consisting of two rows of flat lamella.'., attached to a single 

 stem, they have recently been regarded as forming a special order 

 (Protobrauchiata). 



This group is of special interest because of its great antiquity. 

 Large numbers of fossil forms very closely allied to existing genera 

 and species occur even in Silurian and Devonian formations. 



Thus the common living genera Nucula and Leda are represented by 

 numerous Devonian species, many of which can not be separated from 

 the recent forms, even as subgenera, by any tangible characters. Other 

 species of the same age, referred to Palwoneilo, agree in nearly all essen- 

 tial characters with the living genus Tindaria. These fossil shells are 

 generally larger and stronger than the corresponding living species. 

 Many Paheozoic genera which are now extinct were as highly organized 

 and as much specialized as their living allies. 



The thin-shelled, strongly siphonate genera, such as Yoldia, Yoldi- 

 ella, etc., do not appear so early in geological time and may be regarded 

 as more modern specializations of the Leda-like forms. They are also 

 the forms that swim and jump with the greatest activity. Therefore 

 the thin and light character of their shells may be regarded as having 

 been secondarily acquired, partly in consequence of their active move- 

 ments, in which a heavy shell would be disadvantageous, and partly 

 because the development of long siphons enables them to live concealed 

 much of the time beneath the surface of the soft mud in which they 

 generally live. In Solemya the shell is still lighter and thinner, in 

 accordance with more developed swimming habits, combined with bur- 

 rowing when at rest. Such forms as Nucula and Tindaria, which have 

 no siphon tubes, must live at or near the surface of the nmd, over which 

 they creep with their large expanded pedal disk. These have, for their 

 protection, comparatively solid shells similar to those of Pakeozoic spe- 

 cies, in form, texture, and sculpture. 



The family Nuculidai differs fromLedida3 mainly in having no siphon 

 tubes, the mantle edges being completely disunited. The Ledida^ are 

 remarkable for the great variations in the structure of the hinge-teeth, 

 ligament, cartilage, and mantle, as well as in the form of the shell. 

 The pallial sinus may be wanting or well- developed. Some genera 

 have long united siphons ( Yoldia) ; some have shorter ones, more or less 

 separated (Leda); while in Tindaria there is no true siphon, but only au 

 Proc. IS". M. vol.. xx 



