white] ANNOTATED CATALOGUE. ] 9 



It is not improbable, therefore, that representatives of the Unionida? ex- 

 isted as early as the later Devonian, as has been suggested by Professor 

 Hall. This opinion, furthermore, is all the more probably correct, be- 

 cause of the known fact that a well developed land-inolluscan fauna 

 existed during the Coalmeasure period, and apparently also in the De- 

 vonian, all the known species of which are mentioned on the following 

 pages and figured on accompanying plates. Although it may not be 

 regarded as certain that the Cypricardites cattslcillensis and G. angustata 

 of Vanuxem really belong to the genus Anodonta, copies of Vanuxeurs 

 original figures are given on Plate 1, for the purpose of comparison. 



Dr. J. W. Dawson, in his Arcadian Geology, second edition, describes 

 seven species of edentulous bivalves from the Coalmeasure strata of 

 Nova Scotia, under the generic name of Naiadites. He states his belief 

 that they are the shells of brackish-water or freshwater mollusks " allied 

 to the Mytilida? or Unionida?." The shell structure, as given by Dr. Daw- 

 son, is like that of the Unionida? and unlike that of theMvtilida?. It 

 seems, therefore, not improbable that at least a part of his species really 

 belong to the Unionida?.. Three of those species, namely, Naiadites car- 

 bonaria, N. elongata, and N. hcvis, are represented on Plate 2, the fig- 

 ures being copies of Dr. Dawson's original illustrations in the work 

 referred to. 



If these Devonian and Carboniferous shells do not really belong to the 

 Unionida?, the earliest known members of that family now known to 

 exist in North American strata are two or three species, specimens of 

 which were collected by Prof. E. D. Cope in the valley of Gallinas Creek, 

 New Mexico, from strata Avhich he regarded as of Triassic age.* These 

 shells belong unquestionably to the genus Unio proper, as is shown by 

 the character of the hinge and the muscular markings. The outer pris- 

 matic layer of the shell which characterizes the Unionida? is also well pre- 

 served on some of the specimens. One of these forms, which is figured 

 on Plate 3, was described by Mr. Meek,t under the name of U. cris- 

 tonensis, and specific names were also proposed for the two other forms, 

 the specimens of which he deemed to be too imperfect for characterization 4 

 There are some reasons for regarding the strata from which these shells 

 were obtained as of Jurassic instead of Triassic age, but further inves- 

 tigation is needed before such an opinion can be confidently expressed. 

 The figure of U. cristonensis is drawn from one of the best of Mr. Meek's 

 type specimens, but which is nevertheless very imperfect. The species 

 has never before been figured. 



While U. cristonensis is probably the most ancient published North 

 American species of Unio, this portion of the subject ought not to be 



* An. Rep. Expl. and Sur. -west of the 100th meridian, for 1875, p. 81. 



t An. Rep. Expl. and Sur. west of the 100th meridian, for 1875, p. 83. 



t These two names are respectively Unio gallinensis and V. ierrm-ruorw ; hut the spec- 

 'niens to which they are applied are really too imperfect to justify the application of 

 any specific names. 



