112 SUPPLEMENT TO THE 



elortgata the ventral valve is, if anything, slightly more convex than the dorsal, and 

 consequently the spirals are slightly more convex on the ventral side, and the length of 

 the coils on that side is still further increased by the notch or indentation on the ventral 

 slope of the posterior border of the spirals. Again, as we shall see in the sequel, in 

 Atrypa marginalis, A. Barrandei, and young specimens of A. reticularis, the dorsal 

 valve is nearly or quite flat ; and this being the case there are several differences 

 between their spirals and those in the full-grown specimens of A. reticularis, the dorsal 

 valve of which is ventricose. First, there is a less number of spiral coils ; but to allow 

 space even for these some changes were necessary in the arrangement of the spirals, and 

 therefore the principal coils, instead of being level, are slightly higher on their inner than 

 on their outer sides, whilst, unhke the majority of the full-grown specimens of A. 

 reticularis, the principal coils are some little distance apart, and the ends of the spirals 

 bend over to meet each other. 



It will be seen at once that these peculiarities in the arrangement of the spirals (the 

 coils being only few in number) make them on their dorsal side to be almost level, and 

 suitable, therefore, to the interior space which they have to fill. 



Atrypa reticularis is one of the most abundant fossils in the Wenlock and Ludlow 

 rocks of England. 



11. Atrypa aspeeula, I)av. Sil. Mon., PL XIV, fig. 22 ; Sil. Sup., PL IV, figs. 8, 8 a. 



Atkypa reticularis (Linn.), var. Davidson. Sil. Mon., p. 129, 1867. 



Shell small, transversely oval or wider than long ; valves equally and moderately 

 convex, with sometimes the indication of a small mesial elevation or large central rib 

 in the dorsal and a corresponding small depression in the ventral valve ; beak 

 very small and but slightly produced, increased and truncated by a small circular 

 foramen, laterally margined with small deltidial plates. Surface of both valves covered 

 with from eight to eleven large, strong, radiating ribs, with interspaces of about equal 

 breadth; the valves are likewise crossed by numerous regular projecting concentric 

 lamince, to some extent overlapping each other, and giving to the valves a strong 

 imbricated appearance. 



Length 5, breadth 6, depth 3 lines. 



0^5.— When describing Atrypa reticularis, at p. 130 of my ' Silurian Monograph,' I 

 considered this little shell as a variety of Linne's species. Since then Mr. Maw has 

 collected a considerable number of specimens of this shell from the debris of old 

 Wenlock Limestone quarries at Benthall Edge, and these have enabled me to see that it 

 differs materially and constantly from A. reticularis by its small dimensions and 



