480 DR WHEELTON HIND ON 



more numerous in the DeA^onian rocks. Several genera persist into Carboniferous 

 times, but in them many genera are represented by fewer species than obtained in the 

 Devonian period. 



The most striking feature of the Ordovician lamellibranch fauna, when compared 

 witli those from the Devonian and Carboniferous rocks, is the absence of shells 

 belonging to the Pecten family. Two species only seem to me to be in any way 

 related to the Pectinidse, and for them I have erected the new genus Protopecten. 

 The Aviculidae, on the other hand, are represented by the genera Pterinea, Goldfuss, 

 and Leptodesma, Hall. The number of genera having modioliform or mytiloid 

 characters is large, but these, though modioliform in shape, have, in most cases, well- 

 developed cardinal and lateral teeth in the hinge plate. The Nuculidse are represented 

 by Nucula, Nuculana, Ctenodonta, and Palasoneilo, the last two apparently dying 

 out in Carboniferous times. The nuculiform type of hinge is, therefore, very old 

 and remarkably persistent. The type of hinge developed in Cyrtodonta, Billings, 

 persisted with very slight change in detail to Cretaceous times, being represented in 

 Mesozoic rocks by Grammatodon, Meek and Hayden. That curious genus 

 Conocardium shows the same peculiar habits of growth in Ordovician times that 

 it does in its last representative of the Carboniferous period. One cannot help being 

 struck by the remarkable degree of stability shown by the class of lamellibranchs 

 through all ages, and not only in the persistence of type, but also in the permanence 

 of structural detail. I have not been able to assure myself that any of the Ordovician 

 lamellibranch genera were sinupalliate, and surface ornamentation is, as a rule, much 

 simpler than obtains in Devonian and Carboniferous species. Very few lamellibranchs 

 have been described from rocks older than Ordovician. Hicks figured and de- 

 scribed in 1873 twelve species referred to five genera, from the Tremadoc series of 

 St Davids (op. supra cit.), the genera Ctenodonta and Modiolopsis and Palsearca 

 (Cyrtodonta) being represented; and others have been described by Walcott 

 from the Olenellus zone of North America. Peach has recorded, in the Geological 

 Survey Memoir on "The Geological Structure of the North- West Highlands of Scot- 

 land," the presence of one genus of lamellibranchs. 



Salter described a series of derived lamellibranchs found in the Triassic pebble 

 beds of Budleigh Salterton, the exact age of which is doubtful. Certain genera, we 

 thus learn, were in being in Cambrian times, and nothing whatever is known of their 

 ancestors. In the examination of the fine series of lamellibranchs in Mrs Gray's 

 collection, I have not been able to learn anything as to lines of descent of the various 

 genera. The great lesson gained is the knowledge that many genera have been in 

 existence for immense periods of time. 



The general succession and subdivisions of the Girvan series, as worked out by 

 Professor Lapworth, are given in the following table, from which it will be seen that 

 there is a marked divergence in the lithological characters of the strata from those in 

 the central Moffat region of the south of Scotland. In most of the subdivisions there 



