T. Davidson — Upper Silurian Brachiopoda. 147 



able to exaraine upwards of eight thousand specimens of the species 

 from the Wenlock and Ludlow rocks of Shropshire, I have ascer- 

 tained how much it varies in shape, and especially so at different 

 stages of growth. The smallest or youngest examples measured not 

 much more than half a line in length and breadth, and every stage 

 was obtained up to shells measuring one inch and a half in length 

 and breadth. When quite young the dorsal valve is flat or nearly 

 so, with a strongly-marked longitudinal mesial depression; this 

 same valve with age becomes gradually more and more convex 

 or gibbous, and loses gradually all trace of the longitudinal 

 depression. 



The front line is also either straight or more or less curved 

 upwards, so much so that many specimens show in the dorsal valve 

 a well-developed mesial fold, with a corresponding sinus in the 

 ventral one. The ribbing varies also to a very considerable extent 

 in different specimens. In young individuals the ribs are few in 

 number, and in this condition it much approaches in shape and 

 character to similar-sized examples of Atrypa Barrandi. The 

 number of ribs seems also to increase rapidly with age. Some 

 specimens with very convex dorsal valves are covered with numerous 

 fine radiating ribs, while others of the same size show a much 

 smaller number, and these more coarse and prominent. The 

 concentric lines or squamose ribs due to growth are also much 

 stronger, closer, or more wide apart in some individuals than in 

 others, still all these individuals are linked one to the other by 

 gradual passages. Feeling anxious to ascertain whether there 

 existed interiorly any gradual increase in the number of spiral 

 coils from the young up to the adult condition, 1 placed in the 

 hands of the Eev. N. Glass a number of well-preserved specimens 

 at different stages of growth, and some of which he kindly developed 

 with his usual ability, and he was soon able to show, and in the 

 most distinct manner, that the number of coils in each of the 

 vertical spiral coils increased with the growth of the shell. 

 In a specimen measuring four lines in length and breadth 

 there were only five convolutions in each spiral cone, and these 

 specimens much resemble those of Atrypa Barrandi, and in all 

 probability, if not certainly, in still younger specimens Mr. Glass 

 would have found not more than three or four coils. 



In a specimen measuring five lines in length he found six coils, 

 in another six lines in length seven coils, and so on no doubt the 

 increase would proceed up to fifteen convolutions in each spiral — 

 the usual number found in full-grown specimens. Mr. Glass ascer- 

 tained likewise that the basis of the spiral cones in young specimens 

 with flattened dorsal valves is not level, the two inner sides 

 of the principal coils being slightly higher than the two outer sides, 

 and turned towards the margin of the shell — which is exactly what 

 we have described and represented in Atrypa marginalis and Atrypa 

 Barrandi, the dorsal valves of which are also nearly or quite flat 

 (see pp. 10 and 11 of this paper). As the shell grows, and the 

 dorsal valve becomes more convex, the basis of the spirals becomes 



