292 T. Davidson — On the Genera Merista and Dayia. 



■mesial ridge forms a longitudinal groove, the muscular impressions 

 being slightly in relief on either side. The sockets are wide!}'- 

 separate. The primary stems of the spirals are attached to the hinge- 

 plate of the dorsal valve, and after extending pai'allel 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 the front they form 

 four or five convolutions, 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 lamellaa give off two processes, 

 which converge and extend between the spii'al coils in an upward 

 and backward direction. 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 Prof. J. Hall in Zigospira, but in this last named genus 

 the spiral coils have their extremities facing each other in the centre 

 of the shell, while in Dayia 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 are situated the 

 articulating tooth ; under these are two obliquely placed or chevron- 

 like, elongated, oval-shaped muscular scars, considerably raised from 

 the bottom of the valve, these projecting parts forming correspond- 

 ing depressions in the internal cast. 



We are therefore now, thanks to the incomparable skill of the 

 Eev. Norman Glass, fully acquainted with the characters of the spiral 

 arrangements of this remarkable genus, and which I name after the 

 Eev. H. G. Day, in considei'ation of the important help he has 

 always been ready to offer me in my investigations of the Silurian 

 fossils with which he is so well acquainted. Placed by Sowerby in 

 1839 with Terehratula, by M'Coy in Atry-pa in 1846, with Hypotliyris 

 by Phillips in 1849, with Hhynchonella by Salter in 1859, I hope it 

 has now found a resting-place in Dayia, being entirely dissimilar 

 from any of the genera above quoted. Dayia navicula seems con- 

 fined to the Upper Silurian. It would be very desirable that the 

 interior of the so-termed Merista ? cymbula should be examined, for 

 it bears much external resemblance to Dayia navicula. 



From the different articles we have inserted in this year's volume 

 of the Geological Magazine, it will be seen how very important it 

 is to become acquainted with the loops and attachments of the sjjirals 

 in the spiral-bearing genera of Brachiopoda. Indeed, it has been 

 demonstrated from the admirable researches, so skilfully conducted 

 by the Eev. Norman Glass, that it is impossible to feel certain as to 

 the genus to which the larger number of the spiral-bearing species 

 really belong until their interior details have been ascertained, and 

 how fallacious it is to depend solely on external appearances. 



We are, unfortunately, not yet acquainted with the internal 



