194 PALEONTOLOGY OF NEW YORK. 



the Liorhynchus Umitaris, Vanuxem, frequently constitute entire strata of some 

 inches in thiclcness; and also, in the more bituminous layers of the Hamilton 

 group proper, L. muUicostus, Hall, and at times, L. quadricoslatus, Hall, become 

 very abundant, and are correspondingly rare as the shales lose their organic 

 matter and become more calcareous. In the black Genesee shales, L. quadri- 

 costatus, Hall, is often abundant. Where the succession of the sediments was 

 more persistently calcareous or ai'enaceous the shells adapted themselves to 

 their surroundings, though under such circumstances not attaining so great 

 development in individuals. In the calcareous layers of the Hamilton group 

 at Thedford, Ontario, L. Laura, Billings,* is not of infrequent occurrence. L. 

 Kelloggi, Hall, occurs in the upper Devonian calcareous sandstones of northern 

 Ohio ; L. mesacostalis, L. sinuatus, Hall, and L. globuUformis, Vanuxem,f in the 

 sandstones of the Chemung group ; the L. Newberryi, Hall and Whitfield, from 

 the Erie shales, of Devonian age ; the L. Greenianus, Ulrich, from the Knobstone 

 formatiou of Keokuiv age, and the L. Boonensis, Shumard, in the Burlington 

 limestone. 



In the later representatives of this subgenus there is a tendency to obsoles- 

 cence of the plications over the entire surface ; and in all specimens where the 

 interior is well preserved, the muscular impressions of the brachial valve form 

 narrow, elongate-oval scars alongside the median septum. Frequently, also, 

 the narrow pit beneath the hinge-plate supported by the median septum, is 

 of conspicuous size, as in L. globuUformis. The significance of the group of 

 fossils embraced by the foregoing divisions, Camarotcechia, Plethorhynchus 

 and Liorhynchus cannot be gainsaid. The existence of an incipient spondyl- 

 ium between the divisions of the hinge-plate, supported by the median septum, 

 at once indicates a relationship, not so much to the pentameroids, which have 

 for the most part preceded these in time, but to the spondylium-bearing shells 

 of the later palaeozoic periods, Camarophoria and its allies. 



* Some of the moi-e oblate forms of this species seem indistinguishable from L. muHicosLus, Hall, but 

 L. Laura noi-mally has an elongale-oval outline which is not possessed by typical examples of the former. 



t Before us is a specimen of the Rhynchonella castaiita. Meek, fi-om the Eureka District of Nevada, 

 agreeing with Mr Walcott's identification of this species as described in volume viii, Monographs of the 

 U. S. Geological Survey, p. 153. This specimen demonstrates a very close specitic similarity to Liorliynchns 

 givhulifonnis, Vanuxem, and serves to fix its geneiic relations beyond doubt. 



