TKEXTOX CONGLOMERATE OF RYSEDORPH HILL 23 



in the surface ornamentatioii, composition of the cardinal process, 

 arrangement of the muscle scars, and specially in the great mus- 

 cular scars and their high walls. Three species are cited as 

 clearly referable to this genus, viz Christi.ania subqua- 

 drata Hall, from the Helderbergian (?) group of Tennessee, 

 Chris tiania tenuicincta McCoy sp. from the upper 

 Llandeilo and the Caradoc series of Great Britain, and Chris- 

 tiania o b 1 o n g a Pander sp. from the lower Siluric beds of 

 the vicinity of St Petersburg. 



A comparison of our species with the descriptions and figures 

 of these three species brings out the fact that it is more closely 

 related to the two European lower Siluric species than to the 

 American Devonic; for the general form, it seems, is of little 

 value for a comparative study, as both the English and the Rus- 

 sian formis vary greatly between s^hort, broad and elongate forms, 

 as is fully demonstrated by the figures given by Davidson ^ and 

 by Murchison, Verneuil, and Keyserling ^ and also by the writer's 

 specimens. While, however the Devonic form has the cardinal 

 angles rounder and little produced, the surface smooth or only 

 marked by squamous growth lines and the external lateral wall 

 of the anterior adductor muscle in the brachial valve inflected 

 in front of the crural processes, the British and Trenton species 

 have rather strongly developed cardinal extremities, and the 

 Russian and the Trenton forms have the longitudinal surface 

 striae in common, for Murchison, Verneuil, and Keyserling report 

 that specimens occur which have a striation like Leptaena 

 s e r i c e a . Judging from the figures given in the Geologle de la 

 Russie d/Europe the external walls of the anterior adductor 

 muscles in the Russian form are straight continuations of the 

 crural processes as in the Trenton form. Perhaps, however, to 

 the latter character no great importance should be attached, as 

 Davidson figures a brachial valve of Christian! a tenui- 

 cincta with just such straight lateral walls among the other 

 valves with inflected lateral walls, such as the Devonic form has. 



1 Monograph Brit. foss. Brach. Sll. suppl. 1833. v, 5, pt 2, pi. 12, flg. 17-21, aucl v. 7, no. 4, pi. 47, flg. 

 7-18. 



Qeol. de la Russie d'Europe etc. 1845. pi. 15, flg. 2a-f. 



