NOTES ON THE BEITISH JURASSIC BRACHIOPODA. 21 



of forms, that one is continually at; a loss to draw even 

 a conventional line between two closely allied so-called 

 species" (10, p. 288). 



To this we may also add, that not only is it difficult to 

 draw a line of demarcation between two closely allied 

 species, but it is sometimes equally difficult, from exter- 

 nal appearances alone, to separate the individuals of two 

 well-defined and established genera — Magellania (Wald- 

 heimia King) and Terehratula to wit. 



In the character of their ornamentation, too, the Bra- 

 chiopoda vary very considerably. 



Among the fossil forms we occasionally meet with some 

 which have retained their original colour-markings, as 

 Dielasma (Ter.) hastata, and Discina nitida from the Car- 

 boniferous ; Ter. intermedia from the Cornbrash ; Ter. 

 hiplicata from the Upper Greensand ; and Zeilleria ? (Wold.) 

 perforata from the Lias. A specimen of the last-named 

 form, showing colour-markings, which I found in the 

 Lower Lias of Stout's Hill, Bitton, was figured and de- 

 scribed by my friend, Mr. E. Wilson, F.G.S., and is now on 

 view in our city museum. It is a very beautiful specimen, 

 and shows how handsome some of the fossil forms must 

 have been (21, p. 458). 



Another noticeable feature in the Brachiopoda is the 

 great difference in the thickness of the shell; some being 

 almost semi-transparent and fragile, such as Lingula, etc , 

 while others attain to very near an inch in thickness — e.g. 

 the ventral valve of Prodncttis Uangollensis, Carboniferous. 



Hitherto I have alluded to the shell of the brachiopod as 

 if it were only a single shell ; as a matter of fact it is 

 double — the soft parts of the animal being protected by a 

 shell composed of two distinct pieces or valves, which are 

 practically symmetrical or equal-sided ; hence they are said 



