194 PALEONTOLOGY OF NEW YORK. 



the Liorht/nchus limitarts, Vanuxem, frequently constitute entire strata of some 

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

 group proper, L. multicostus, Hall, and at times, L. quadricostatus. 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 arenaceous 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 Ccalcareous 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 

 formation of Keokuk 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 more oblate forma of this species seem indistinguishable from L. mvlticostua. Hall, bat 

 L. Laura normally has an elongate-oval outline which is not jwssessed by typical examples of the foimer. 



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

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

 U. 8. Geolo-iical Survey, p. 163. This specimen demonstrates a vei-y close specific similarity to Liorhynchus 

 glol/tUybrmis, Vanuxem, and serves to fix its generic relations beyond doubt. 



