part 2] SYHINGOTHTRIS AND SPlRlFERrNA. 177 



(2) The curvature of the area. — Although exhibiting- a fairly-wide 

 range of variation, the nature of the area in forms from one horizon, and 

 in rocks formed under similar conditions, is fairly constant. 



In common with Spirifer, Spiriferina, and similar forms, the cardinal area 

 is marked by numerous parallel transverse striations, which in suitably- 

 preserved specimens are seen to be continuations of the concentric lines of 

 growth that appear on the remainder of the surface of the shell, and therefore 

 have a direct bearing upon the height and curvature of the area. The addi- 

 tion of new shelly matter in the Brachiopoda takes place at the free edges of 

 each valve. 1 



If the deposition along the hinge-line is, throughout life, less than along 

 the anterior margin of the shell, the area will be strongly curved, as in most 

 species of Spirifer : as the marginal growth becomes more nearly uniform the 

 area increases in height and decreases in curvature ; while, when the growth. 

 along the hinge-lines is in excess, the area will be reclined, that is, convex. 

 Usually, in the genera under consideration, the rate of growth along the 

 hinge-line either increases or decreases progressively, during the life of the 

 individual, so that, although in its apical portion the area is curved, it 

 becomes practically flat towards the hinge-line. 



In a general way, the curvature of the area is of taxonomic value, but 

 it sometimes happens that in one and the same specimen the area is convex 

 on one side of the delthyrium, and concave on the other side (see PI XI, 

 fig. 7). 



The area, although normally symmetrical, is sometimes distinctly twisted ; 

 but that feature depends upon differences in the curvature of the area on 

 either side of the delthyrium. 



The variations in the height of the area are, of course, accompanied by 

 changes in the profile of the pedicle-valve, and the curvature of the floor of 

 the sinus along a median line from the beak to the anterior margin is, espe- 

 cially in imperfectly-exposed specimens, a useful diagnostic character, since 

 from it the nature of the area can be inferred (see fig. 3, p. 178). 



(3) The lateral slopes. — The lateral slopes may be either flattened or 

 tumid, and from the beak to the anterior margin may be either concave or 

 convex according - to the curvature of the area, being convex when the area 

 is concave, and concave when the area is reclined. Excessive obesity is a 

 gerontic condition. 



(4) The costs;. — The costal vary in size, in abundance, and in their rela- 

 tion to the furrows which separate them. They are sometimes nearly or quite 

 obsolete, particularly upon the lateral slopes of the ventral valve. Obsolete 

 costas are usually accompanied by a reclined area. 



(5) The transverse plate and syrinx. — These are useful diagnostic 

 features. The transverse plate varies in its greatest depth beneath the plane 

 of the cardinal area, and the syrinx varies in size relatively to the transverse 

 plate ; but, as the variations will be dealt with when certain typical forms are 

 described, further details need not be given here (see fig. 1, p. 166). 



(6) The minute surface-ornament and microscopic struc- 

 ture of the shell. — In the majority of specimens the intimate structure 

 of the shell is obliterated by recrystallization, or the outer layers are lost 

 by exfoliation, so that these features are seldom available for diagnostic 

 purposes. 



1 J. F. van Bemmelen, ' Untersuchugen iiber den Anatomischen & Histolog- 

 ischen Bau der Brachiopoda Testicardinia,' Jenaische Zeitschr. vol. xvi (1S83) 

 p. 465. See also ' Cambridge Natural History : Mollusca & Brachiopoda ' 

 1895, p. 468. 



