T. Davidson — On the Genera Merista and Daijia. 291 



valve and with their narrow end fitting under the extremity of the 

 beak. Prof. King compared these plates to a shoe-lifter process. 



With very small differences the loop of Meristella, Hall {M. 

 arcuata, Hall), is similar to that we have described in Merista, and 

 were it not that Meristella has no shoe-lifter process, it would not be 

 possible to distinguish the two genera. 



Again, Whitfieldia {Atrijpa tiimida of Dalraan) is distinguishable 

 from both Merista and Meristella by the absence of those peculiar 

 ring-shaped processes attached to the loop, and has instead only a 

 short bifurcating process where in both the last-named genera the 

 rings are formed. These three genera seem, indeed, closely allied, 

 although each contains peculiarities by which it may be distinguished 

 from the others. 



Genus Dayia, Dav. 1881. 

 Type Terebratula navicula, J. de C. Sowerby, Sil. Syst., pi. v. fig. 17, 1837. 

 At page 191 of my Silurian Monograph, I say, " Surely this shell 

 differs much, both by its external as well as its internal characters, 

 from those peculiar to the genus Bhynchonella : so much so that it 

 may hereafter be found desirable to propose for it, and similarly 

 characterized shells, a separate generic or sub-generic desig- 

 nation." In 1867 I was acquainted with the interior surface of 

 both valves, and described and figured in detail its very remarkable 

 muscular and other impressions ; but I had no idea that the shell was 

 provided with spiral coils for the support of the labial appendages. 

 During the month of March, 1881, the Eev. H, G. Day showed me 

 some fine specimens of the so-termed BhyncJionella ? navicula, and 

 offered to send them to the Eev. Norman Glass, that he might see 

 whether the shell was possessed of spiral appendages, and on ' the 

 22nd of the same month Mr. Glass wrote me : " I now send you 

 two specimens worked out of B. ? navicula, showing entirely new 

 spirals and loop," and since all the interior characters are so distinct 

 from what we find in other spiral - bearing genera, Mr. Glass 

 suggested that I should propose a new genus for the shell under 

 description. It is very probable even that we have in our British 

 Silurian rocks other species referable to the same genus, but we are at 

 present acquainted with Dayia navicula only, 

 so that the generic characters may be taken 

 from that as the type. Exteriorly, Dayia 

 navicula is elongated, oval or boat-shaped, 

 broadest posteriorly — ventral valve very 

 deep, convex, and arched, and keeled along 

 the middle, beak closely incurved, dorsal 

 valve slightly convex posteriorly, anterior 

 half of shell concave, surface smooth. In 

 the interior surface of the dorsal valve a 

 slightly raised ridge extends from under the 

 hinge-plate to about half the length of the 

 Interior of the dorsal valre of valve, and on either side are two scars 

 Bayiatiavieula. Developed f(jj.^j^g^ ^ ^^^^ adductor muscle. On the 

 by the Eev. N orman Glass. . , ^ i. ^.^ ^ • j i . i 



•' internal cast the place occupied by the 



