130 



causes is not so obvious, nevertheless the variability of the species is 

 evident on examination of the accompanying tables. Ten valves from 

 Dugald brook show an average of 5 p.c. excess of length, and seventeen 

 valves, from Boundary brook an average of 9 p.c. The attitude of the 

 valves has not affected their form in the same way as it has the ventral 

 valves, as was explained in the remarks on orientation, and it may be 

 assumed that the natural width and length of the dorsal valves of this 

 species were about equal to each other. 



Another fact noticeable in this connection is that only a few valves 

 reach the full adult size. Also the shells from Dugald brook grew to a 

 larger size than those from Boundary brook, where the measures of Assise 

 E. 1 d. are more sandy. This is an unexpected result, as in later Camb- 

 rian terranes Lingulepis has been found especially to characterize sand- 

 stones. But it agrees with the habit of this particular species which, 

 while rare and small in the earlier and more sandy assises of Division 1, 

 became plentiful when the mud beds of Assise E. 1 d. were laid down 

 (see annotation of species in the several assises p. 78). 



Resting stages Cicatrices or resting stages in Lingulepis Gregwa. This species was 

 apt to develope concentric grooves or cicatrices at certain stages of growth. 

 Thus of eight ventrals from Dugald brook three show strong cicatrices 

 at 7 mm. from the umbo and all show them at 9 mm.j but not so distinctly. 

 Of thirteen ventrals from Boundary brook twelve show grooves at 6 mm. 

 from the umbo and eight show them, but not so well marked, at 8-9 mm. 

 Of ten dorsals from Dugald brook, eight show a groove usually strong, 

 at about six mm. from the umbo, and four show one at 8-9 mm. Of 

 fifteen dorsal valves from Boundary brook, eleven show a groove, often 

 strong, at about 6 mm. from the umbo, and three show weak ones at 8-9 

 mm. from that point. At both localities a few ventrals show resting 

 grooves at 4 mm. from the umbo. 



Allowing one to two millimetres for the extra length of the ventral valve 

 these cicatrices on the two valves would come opposite each other. The 

 d,eep ones are apt to be placed just outside of the visceral cavity of the 

 dorsal valve, therefore considerably beyond the point of attachment of 

 the central group of muscles of the ventral valve. The corrugation caused 

 by these cicatrices would, no doubt, serv;3 to strengthen these thin shells ; 

 yet we find some that have been decollated at this line, as the band of the 

 visceral callus in a few valves has been found to extend quite to the front 

 of the valve. 



Mr. Walcott has made some curious mistakes in regard to this species.* 

 First he places it as a species of the Paradoxides beds, when it actually 



*Cambrian Brachiopoda, &c., Bull. U.S., National Museum, vol. xxiii, pages 692-694. 



