294 GENERAL SUMMARY TO 



portions of the valves displayed perforations, whereas others from the Province of Arch- 

 angel, in Russia, had some parts of the valves quite imperforate ; and he explains these 

 differences to be due to the shell-tissue in the latter specimens having become silicified 

 or otherwise modified. This occurs very often, and I have noticed it repeatedly to be 

 the case in a number of Palaeozoic specimens, the real structure of the shell having been 

 altered through fossilisation. The punctures in Rhynchopora Youngii (Dav.), says Mr. 

 Young, can be seen with the hand-lens, and still better under the microscope. 



The punctures vary much in different genera and species, and this circumstance has 

 been often alluded to in the pages of this Monograph. The shell-punctures in Orthis 

 Mickelini are very remarkable and have been described and illustrated at p. 293 of my 

 ' Carboniferous Supplement.' There we have, according to Mr. John Young's investi- 

 gations, first, a series of very numerous and minute microscopic perforations or canals that 

 reach the inner surface of the shell, and traverse the whole thickness of the valve ; then 

 a second series of pores, larger and less numerous than those of the first series ; lastly, a 

 third series, which is at least twenty times larger and stronger than those of the second 

 series, and which belong to the basis of the larger broken spines. Mr. Young ascertained 

 likewise that the pores of the second and third series, referable to the spines, only enter 

 in a slanting direction for a little distance into the substance of the shell. 



At my request Mr. Young made a minute examination of the shell-structure of 

 Produclus and Chonetes ; and this has been fully given at pages 295 to 302 of my 

 'Carboniferous Monograph' (vol. iv), and in the 'Appendix to the Supplement,' page 280, 

 antea (vol. v). Mr. Young ascertains that the perforations do not in these genera nor 

 in Streptorhynchus, Strophomena, and Leptcena, pass through the entire substance of the 

 shell, but that in Productus they enter from the inner surface of the valves and gradually 

 get smaller and smaller as they pass outward through part of the substance of the shell, 

 and are lost before they reach the outer surface. In Chonetes Mr. Young says the inner 

 series of perforations seen on the interior surface of the valves are very much after the 

 style of those visible on Productus; but that observers may be deceived when the 

 specimens they are examining have been decorticated or have had the outer surface of 

 the shell eroded by weathering. On specimens of Chonetes Laguissiana ; De Kon., 

 where the outer surface of the exterior of the valves is well preserved, he has observed 

 also a series of minute perforations on the ribs, but which penetrate to a very small 

 distance only. In the species of this genus, as in Streptorhynchus, Strophomena, and 

 Leptcena, Mr. Young found that the perforations in all these genera are quite different 

 from those of Terebratula and Orthis, which pass through the whole thickness of the 

 shell. In the former the perforations slant inwards and upwards from the interior 

 towards the beak of the shell. The laminae of the shell also show, as Dr. Carpenter 

 terms it, an infundibular arrangement around the punctures. The interior surface of 

 the valves of Streptorhynchus is closely and strongly punctate or rather pustulate. 



The shell-substance of Linyula Professor King believes to be almost entirely com- 



