492 



DR F. A. BATHER. 



Theca. 



Pbriproct. 



Ratio 



22-5 



14-5 



•64 



20 



15-1 



•75 



20-5 



16-2 



•79 



15-5 



12 



•77 



17-3 



13-7 



•79 



25-5 



20 



•78 



27 



22-7 



•84 



23-5 



20 



•85 



24 



20-5 



•85 



This is expressed in the frame by 



diameters of theca and periproct, measured in millimetres at the level of plates 6 and 9 

 in various species : 



P.Jilitexta (Billings' figure) 



P.Jilitexta (El 6046) . 



P. squamosa (Billings' figure) 



P. squamosa (E 16036) 



P.foriolus . 



P. gibha 



P. anglica . 



P. Rugeri , 



P. quadrata 



Secondly, the periproct is extended adorally. 

 the separation of plates 12 and 14, which, if ever they meet, do so only by their 

 extreme points; as a rule they are wide apart, plate 12 abutting on 17 and 23, plate 

 14 on 19 and 18. This results in a weakness that is compensated with difficulty, 

 though an attempt to counteract it may perhaps be seen in the wider shoulder-angle 

 and the strengthening of its surface-folds. But even in the American fossils, where 

 plates 12 and 14 meet in the middle like the two sections of a cantilever arch, this is 

 seen to be the weakest part of the fossil, and the ends of those plates are often broken. 

 Much more is this the case in the British fossils, and in them it is rare for the 

 assemblage of plates covering the oesophagus on the anal face (viz. 23, 18, 24) to be 

 preserved in position. 



Thirdly, the periproct is often stretched downwards to a considerable extent in the 

 rectal lobe, and sometimes, though to a less extent, in the corresponding lobe on the 

 other side of the stem. 



Fourthly, there is an absolute widening of the periproct, due to the increased width 

 of the whole lower half of the theca, correlated with the greater width of plate 5, and 

 its extension towards the left or anal side, and with the increase of the shoulder-angle. 



§ 554. This increase of periproctal area demands greater flexibility, and that is 

 provided for by a tendency to a smaller size of tlie periproctal plates. Roughly speak- 

 ing, those of the American species range from an average diameter of 2*7 mm. in 

 P.Jilitexta and allies to 1 mm. in P. squamosa and allies; those of the British species 

 range from an average of 1 mm. in P. quadrata to 'A in P.foriolus. 



§ 555. There is also observed in the British species a tendency to a shortening or 

 widening of the pectinirhombs. In this respect the British species are not entirely 

 contrasted with the American, since the rhombs are widened in P. squamosa, 

 P. exornata, and P. m,erce7'ensis. In the British forms, however, the process has been 

 carried further ; even the elongate rhombs of P. gihha and P. foriolus are shorter in 

 relation to the suture than are those of P. Jilitexta ; and in such species as P. anglica, 

 P. Rugeri, and P. procera the widening and compression are extreme. In the two 



