96 SUPPLEMENT TO THE 



found in the number of coils in the spirals. Nucleospira, however, differs from the 

 other two in its having the beak-shaped attachments to the hinge-plate, as in Athyris. 



Meristina didyma does not appear to be a very common fossil in our English Upper 

 Silurian rocks ; but it occurs both in the Wenlock and Ludlow series in Great Britain. 



Genus. — Dayia, Davidson, 1S8L 



6. Dayia navicula, Soio., sp. Dav., Sil. Mon., PI. XXII, figs. 20—23; and Sil. Sup., 



PL V, figs. 1 to 4. 



Davidson on the genus Dayia. Geol. Mag., new series, vol. viii, p. 291, 1881. 

 Terebratula navicula, J. de C. Sow. Sil. Syst., pi. v, fig. 17, 1839. 



At p. 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 characterised shells a separate specific, generic, or subgeneric designation." 

 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, 1880, the Rev. H. G. Day showed me some fine specimens of 

 the so-termed Hh. tiavicuJa, and offered to send them to the Rev. 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 R. ? 

 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 certainly 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, arched, and keeled along the middle ; beak closely 

 incurved. Dorsal valve slightly convex posteriorly ; anterior half of shell concave. 

 Surface smooth. On the interior surface of the dorsal valve a slightly raised ridge 

 extends from under the hinge-plate to about half the length of the valve, and on either 

 side are two scars formed by the adductor muscle. On the internal cast the place 

 occupied by the mesial ridge forms a longitudinal groove, the muscular impressions 

 being slightly in relief on either side. The sockets are widely separate. The primary 

 stems of the spirals are attached to the hinge-plate of the dorsal valve, and after extending 



