278 CARBONIFEROUS LAMELLIBRANCHIATA. 



Leptodomus, but am in agreement with that author in acknowledging the propin- 

 quity to Sedgwickia of some of the shells originally referred to that genus. 



Messrs. Meek and Hayden have given an exhaustive summary of the characters 

 and affinities of Sedgivickia (op. sup. cit.) ; the shells which they, however, refer to 

 this genus are much more transverse than any British examples ; and these 

 authors are inclined to refer to the genus several shells of widely different 

 affinities, such as Leptodomus costellatus, M'Coy, Sanguinolites variabilis, M'Coy, 

 and Leptodomus truncatus, M'Coy. With regard to the first two of these, at any 

 rate, there are no grounds whatever for extending the genus to receive them. 



I have grave doubts whether any of the three species included by these 

 authors in the genus really belong to it ; and it would appear that they were 

 themselves in doubt, for their shells are named " Sedgivickia Topekoensis ? " 

 " Sedgivickia ? concava," and " Sedgivickia ? altiro strata." The former of these 

 is shown in the figure to have a well-marked escutcheon and lunule, characters 

 which are not present in Sedgivickia. 



According to M'Coy, the hinge in Sedgivickia is edentulous, and in several 

 species I can see no indications of hinge-teeth in casts, though I must confess 

 that my material was not very suitable for the demonstration of teeth if these 

 were very small ; but in a specimen of Sedgivickia ovata, PI. XXVII, fig. 2, 

 there is an unmistakable cardinal tooth, which is directed obliquely backwards, in 

 the right valve. 



The presence of a small cardinal tooth in at least one species of the genus 

 emphasises the relationship between Schizodus and Sedgwickia which I have 

 referred to above, and may indicate the lines along which these forms evolved ; 

 but more accurate knowledge must be known about the Lamellibranchs of the 

 earlier palaBozoic rocks before anything can be definitely stated on this subject. 



M'Coy does not advance any evidence for his statement re Sedgwickia, " They 

 are entirely without hinge-teeth, their hinge resembling that of Thracia," all the 

 figures being those of exteriors. The statement itself, too, seems contradictory, 

 as the hinge of Thracia possesses a large cartilage socket and ossicle, which are 

 certainly not to be seen in Sedgwickia. 



Sedgwickia gigantea, M'Coy, 1844. Plate XXVI, figs. 1—3 ; Plate XXVII, 



fig. 10. 



Sedgwickia gigantea, M'Coy, 1S44. Synops. Carb. Foss. Ireland, p. 62, pi. xi, 



fig. 40. 

 Dolabka securifoemis, M'Coy, 1844. Ibid., p. 66, pi. xi, fig. 15. 

 Lyonsia secukiformis, d'Orbigny, 1850. Prodrome de paleont., p. 128. 



— Ibid., p. 129. 



