1 6 DAY, Variation in a Cai'boniferons Brachiopod. 



some variation in the strength of the reticulation, since, as 

 Buckman has pointed out, the genus is in the catagenetic 

 stages leading towards ultimate smoothness. The orna- 

 ment of the present species has been described and 

 figured by Davidson (" British Fossil Brachiopoda," Vol. II., 

 Pt. V., p. 225, and PL LI., fig. 15). He says "externally 

 the surface was covered with numerous concentric ridges, 

 rarely in any place more than a line apart, but usually 

 very much closer, and from each of which projected num- 

 erous, closely packed spines, which thus formed a series of 

 spiny fringes overlaying each other all over the shell." 



Very few individuals in the present collection have 

 retained the external skin of the shell, so as to show the 

 well-developed reticulation, but in practically all cases 

 where the skin is lost, the ornament has become impressed 

 on the test beneath, and is still clearly reticulate. PL l.,L, 

 is an enlarged photograph of the outer skin of the test 

 in one very good example, and shows the remarkable 

 character of the ornament. 



The impressing of the surface reticulation on the test 

 beneath, in the present species, is in marked contrast to 

 the features I have observed in other closely similar 

 Reticularis. In these the surface ornament is, as usual, 

 reticulate by combination of radial and concentric orna- 

 ment, but in places where the outer skin has peeled off, 

 only a very strong radial ribbing is to be seen, with no 

 reticulation visible. This radial ribbing represents, pre- 

 sumably, a dominant radial ornament impressed on the 

 test beneath, but in any case, the fact that such an 

 entirely different ornament underlies the true surface 

 ornament, might well lead to the institution of new 

 genera, on what are, in reality, specimens of well-known 

 forms which have lost the outer skin of the test. 



