NORTHUMBERLAND AND DURHAM. 253 



these two fonns run so much into one auotlicr, that it is impos- 

 sible to draw a line of separation between them. 



This species is common in the shell-limestone of Durliam, and 

 occurs also, but more sparingly, in the compact-limestone, and 

 has also a very extensive lateral range. 



2. S. LAMELLOSA, Geinitz. 



In England this shell fs best known as King's S. Morrislana, 

 in Germany it is Geinitz's Orthothrix lamellosus, and Stroph. 

 Cancrim^ de Verneuil. Geinitz's specific name must now be 

 adopted, as, thanks to the untiring perseverance of Mr. David- 

 son, a typical specimen of the genuine Prod. Cancrini has been 

 sent to him from Russia, and it has turned out to be a true 

 Productus. Geinitz's description and figures of Oith. Icmiellosns 

 were published in the early part of April, 1848, and Mr. King's 

 specific name cannot bear an earlier date than the description 

 given in his catalogue, August 19th, 1848. 



This species varies so much witli age and locality, and is so 

 apt to become distorted, that no special form can be given that 

 will include all the individuals which undoubtedly belong to it. 

 In general, young individuals are broader than long — that is, 

 when the shell is of regular growth — and the greatest breadth 

 of the shell is then very little more than that of the hinge- 

 margin. In larger individuals the general form is almost cir- 

 cular, and the hinge-margin appeal's narrower. In distorted 

 specimens the area is much narrower and deeper than in those of 

 regular growth. This is very remarkable in some specimens 

 from Tunstall. At Dalton the specimens are small, and resem- 

 ble the form to which Geinitz has a}>plied the sjiecific name 

 lamellosus. The specimens from Humbleton are much larger, 

 and generally of regular growth. I liave never been able to 

 detect spines on the upper valve of any of the specimens from 

 the above localities, but Mr. Kirkby has lately obtained some 

 individuals from Uyhope with the spines distinctly shown on 

 this valve. These specimens are also rather broader than those 

 from other localities, and the stria^ are nearly obsolete. The 

 most striking cliaracters common to all these forms are the long 



VOL. III. I'T. IV. 1 II 



