BRITISH SILURIAN BRACHIOPODA. 



97 



Interior of the dorsal valve 

 of Dayia navicula. Deve- 

 loped by the Rev. Norman 

 Glass. 



parallel to each other for a short distance bend at right angles abruptly towards the 



lateral portions of the beak, and form two large curves facing the 



lateral portions of the valve. On approaching tlie front they form 



four or five convohitions, which become smaller until they reach 



their terminal coil, which faces the middle of the lateral portions 



of the shell. Near the front the primary lamellae give off two 



processes, which converge and extend between the spiral coils in 



an upward and backward du-ection ; after becoming united 



towards the middle of the shell, they are again prolonged in the 



shape of a single lamella, which proceeds upwards for a little 



distance with its extremity directed towards the hinge-plate. The 



spiral coils are therefore connected by a loop having a somewhat 



similar position to that described by Professor J. Hall in one of his figures of Zygospira, 



but in this figure the spiral coils have their extremities facing each other in the centre 



of the shell, while in Bcajia it is quite the reverse, the extremities of the spiral coils 



facing the lateral portions of the shell. 



In the interior of the ventral valve a mesial groove extends from the extremity of the 



beak to about the middle of the shell, and on either side, running parallel with the hinge- 

 line, are two broad, rounded projections, at the outer extremity of which is situated the 



articulating tooth ; under these. are two obliquely placed or chevron-like, elongated, oval 



muscular scars, considerably raised from the bottom of the valve, these projecting parts 



forming corresponding depressions on the internal cast. 



We are therefore now, thanks to the incomparable skill of the Rev. Norman Glass, 



fully acquainted with the characters of the spiral arrangement of this remarkable genus, 

 which I name after the Rev. H. G. Day in consideration of the important help he 



has always been ready to offer me in my investigations of the Silurian fossils, with which 

 he is well acquainted. 



Placed by Sowerby in 1839 with Terehratula, by M'Coy in Atrypa in 1846, with Hypo- 

 thyris by Phillips in 1849, with BhyncJwnella by Salter in 1859, I hope it has now 

 found a resting place in Bayia^ being entirely dissimilar from any of the genera above 

 quoted. Dayia navicula, as we have stated elsewhere, seems confined to the Upper 

 Silurian. It would be very desirable that the interior of the so-termed Merista? 

 cymbula, Dav., should be examined, for it bears much exterior resemblance to Dayia 

 navicula. The presence, however, of a shoe-lifter process in the ventral valve would make 

 it very doubtful as to the possibility of its belonging to Dayia. 



