22 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



give the shell a plaited look ; these, however, are not true plaits, but undulations, as in 

 fig. 7, which is the most marked example I could find ; in general they are hardly visible, 

 and do not extend to the umbo, as in Sp. Walcotti, Munsterii, &c, and we can trace 

 every stage, from the perfectly smooth specimen to the extreme state represented in fig. 7. 

 The third cause is occasioned by the form of the area and beak, which is very variable in 

 this species ; generally the beak is recurved, as in figs. 1, 2, 4, 5, and 7, allowing one to 

 see under it a deltidium of moderate size ; but as the area and beak become produced 

 and projected backwards (which is the case with many specimens), the beak becomes almost 

 straight or slightly recurved at its extremity, and displaying in all its extent a large 

 elongated deltidium, which is the narrower in appearance as the area becomes longer and 

 larger. The deltidium does not thus widen in comparison to its length ; it would, there- 

 fore, be impossible to separate into species this shell from the form and size of its area and 

 beak, as every insensible gradation can be traced from one shape into another. The figures 

 5, 8, 10, 12 are some examples; fig. 3 exhibits a specimen, in which the beak has become 

 very large and wide. This is a rare case, as well as that seen in fig. 1 ; they are 

 extremes. 



Another cause of variation is due to the punctuation and tubular spines ornamenting 

 the valves ; their size, length, and number being rarely the same in many specimens ; at 

 times being so near to each other that one cannot perceive the intermediate punctuation 

 covering its surface ; in other specimens they are irregularly and sparingly implanted, as 

 may be seen in the enlarged fig. 19, which appearance would seem to have induced 

 M. Buckman to propose one species for fig. 2, under the name of Sp. punctatus, and another 

 for fig. 1, by the name of Sp. rcticulatus ; but in reality there is no difference between these 

 two specimens but in the size of the punctuation and spines, which are larger and stronger, 

 according to the size of the specimens. We also often perceive that much difference appears 

 to have been caused by local conditions ; thus we have full grown adult specimens of three 

 lines in length and breadth, as well as in others of much greater dimensions ; these form 

 varieties in the species, and we might name Spirifer verrucosus of Zieten as a mere dwarf 

 variety of Sp. roslratus. 



The deltidium in this species is also very remarkable, being formed of two pieces united 

 in the form of a roof, which is well displayed in many specimens, especially in those found 

 in Normandy (Vieux pont), which I have represented in PI. II, figs. 2, 3, and 10 ; leaving 

 a passage above the umbo for the peduncular muscular fibres. When the deltidium is not 

 preserved, which is generally the case, the fissure is seen to extend to the extremity of the 

 beak. M. Deslongchamps, Bouchard, De Verneuil, and myself, have several specimens 

 illustrating these points in the most beautiful manner. On both sides of the deltidium 

 extends the area, which is well defined, dividing the beak on each side of the deltidium into 

 two equal portions, at once perceptible by the marked line extending from the extremity of the 

 beak along its whole length, at which line on both sides of the area the spines stop. The 

 remaining portion to the edge of the deltidium being covered by horizontal and vertical 

 lines of growth ; the vertical lines have not, however, much length or regularity, and are well 



