THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 511 



between Ctenodonta, a nuculiform shell, and Macrodon, a typical arciform genus. 

 Hence one cannot but arrive at the conclusion that Ulrich had not been fortunate 

 enough to see the whole of the hinge of Cyrtodonta. 



Salter (op. supra cit.) helped to confuse the synonymy also, giving a hypothetical 

 date to Palsearca, 1857, saying : " Pal sear ca appears to have been in some way in print 

 in 1847. It is unfortunate that it escaped the notice of Mr Billings, as it seems to have 

 done that of all others." This date must also be misprinted, as is the date Salter 

 gives to Cyrtodonta, Billings, 1848 for 1858 (p. 341). It is a pity that Salter did not 

 give the exact reference to the first publication of Palsearca, because Hall discarded it 

 in favour of Cypr {cardites, Conrad, 1841, and it never seems to have been adopted by 

 subsequent American authors. 



Observations. — Mr Salter gave at length his reasons for retaining Palsearca, Hall, 

 in preference to Cypricardites, 1841, Conrad. Miller (N. American Geol. and Pal., 

 p. 476) says : " If the genus can stand, it must be based on this type (C. curtus) (all 

 other specimens are referred to other genera), because this species alone has a hinge- 

 line like the one Conrad made." Palsearca seems to have been dropped in America. 

 Miller places it as a synonym of Cypricardites. I am of opinion that Salter's 

 Palsearca bulla, P. obscura, and P. socialis cannot in any sense be considered to 

 belong to the same genus as P. Billingsiana. 



Palsearca is closely allied to Grammatodon, under which name Carboniferous and 

 Mesozoic arciform shells should now be placed. The hinge plate is very similar in 

 both genera, but the teeth in front of the umbo slope forwards and not backwards. 

 The Ordovician shells are not so carinate as the Mesozoic species, but several Carboni- 

 ferous examples, e.g., G. (Parallelodon) squamifer, G. fallax, and G. Fraiponti, are 

 not carinate. For the present, therefore, the obliquity of the anterior hinge teeth 

 alone distinguishes the genus Cyrtodonta from Grammatodon, and I see no reason 

 whatever to hesitate in placing both genera in the Arcidee. 



Cyrtodonta penkhillense, sp. nov. (PI. IV., fig. 21.) 



Specific Characters. — Shell below medium size, transversely oblong, slightly 

 oblique, inequilateral, obliquely somewhat gibbose. The anterior end is short, its 

 margin rounded. The inferior border very slightly convex ; the posterior obliquely 

 truncate, almost straight, making an obtuse angle with the hinge plate, which is straight 

 and slightly elevated posteriorly. The umbones are small, incurved, placed somewhat 

 anteriorly ; a well-marked subangular ridge passes backwards from the umbo to the 

 postero-inferior angle, and a more rounded shorter ridge marks off the umbo from the 

 small anterior end ; and the dorsal slope is compressed and hollowed. 



Interior. — Not observed in the specimen. 



Exterior. — The surface is almost smooth, but here and there on the body of the 

 valve concentric striae of growth may be seen. 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART III. (NO. 18). 75 



