462 DR F. A. BATHER. 



§ 399. Shape (text-figs. 65-75). Any or all of the three pectinirhombs may be: 

 flush, or surrounded by a border varying in intensity ; elongate along the suture, or 

 widened across it, to varying extents, and in each case occupying either the whole 

 suture or a varying part of it ; long-elliptical, elliptical, broadly elliptical, lanceolate, 

 lozenge-shaped, subrhomboidal, kite-shaped, subtriangular, square, or irregularly 

 trapezoid. To attain precision, the actual ratio of length along the suture to 

 width across it is given where possible, and especially for rhomb 10-14, which is 

 the most generally visible. These shapes, positions, and proportions of the rhombs 

 appear to be valuable diagnostic characters. E. Billings has also noted the surface 

 of the pore-field, whether concave, flat, or convex ; but this is less easy to determine. 

 The number of folds or slits of course varies with the length of the rhomb, but 

 also varies with the coarseness of the folds, and may be regarded as approximately 

 characteristic. Part of a pore-field may be covered in, but this covering starts from 

 the border. 



§ 400. Periproctals, diameter. Variation in this separates groups of species, 

 but several species may have periproctals of almost the same size, and care must be 

 taken to compare periproctals having the same situation. The number of periproctals 

 naturally depends on their size as well as on the extent of the periproct. 



§ 401. Periproct The extension of this and the number of plates that compose 

 its frame are important morphological characters separating groups of species. 

 The chief point is the union of plates 12 and 14, or the passage of the periproct 

 between them. 



§ 402. Ornament. This shows so much individual variation as to be of little 

 value. On the antanal face it consists of: («) rhomb-ridges or folds at right angles 

 to the sutures, and of these the ridges approxiniately bisecting the sutures and 

 connecting the umbones are axial or main ridges, those parallel to them being 

 subsidiary, and being as a rule less strongly marked ; (b) radiate ridges passing from 

 the umbo to the angles of a plate ; it is not probable that subsidiary radiate ridges 

 exist, though they are said to occur in P. mercerensis ; (c) concentric ridges parallel 

 to the sutures. On the stem the ornament is also (a) axial, i.e. vertical or fasciate, or 

 (c) concentric, i.e. transverse. The ridges may be broken up into granules, fine or 

 coarse, regularly or irregularly disposed. The variation consists mainly in the pre- 

 dominance of a or c, and in the greater or less granulation. There is also variation 

 in intensity, and this probably is of less specific value. 



Essentially this ornament is an expression of growth-structure and mechanical 

 forces. The latter are responsible for the rhomb-ridges or " Spannleisten," as Jaekel 

 calls them, which serve to stiff'en the thin plates. The concentric ridges represent 

 lines of growth. The granulation seems to arise by the crossing or interference of the 

 two systems. 



