56 INTRODUCTION. 



Our Accessor!/ cardinal muscles {Cardinalis or hin(/e muscles of Prof. Owen) produce 

 very slight impressions on the internal surface of the shell ; their position is represented at 

 (,2?) in figs. 7, p. 64, and 1, p. 55, but seems to have been omitted in Prof. King's diagram, 

 illustrating the muscular system in Ter. australis. ('English Permian Fossils,' PI. XX.) 



III. The pedicle muscles {retractor superior and inferior of Prof. Owen) have often 

 but not always left impressions in the interior of the valves ; they are wanting in those 

 forms which appear to have lived free, or to have been but slightly attached in the early 

 part of their existence. It is likewise worthy of remark that commonly the peduncle 

 muscular impressions are more clearly defined in those species where the peduncle and the 

 foramen for its passage is comparatively large. The pedicle muscles may be designated as 

 follows : — 



1. Ventral pedicle muscles {retractor inferior of Prof. Owen) form two scars exterior of 

 the adductor, and can be traced in the interior of the ventral valve of all the Terehratulidce 

 in Athyris, Spiri^era, Atrypa, Rhynclionella, &c. 



2. Dorsal pedicle muscles {retractor superior of Prof. Owen) produce two or four 

 recognisable impressions in the dorsal valve, especially in those species provided with a 

 hinge plate, such as Waldheimia, Terehratella} Mayas, Spiriyera,'^ &c. They are likewise 

 observable in a similar position in Mhynchonella, Atrypa, Btrophomena, and some others, 

 where the hinye plate is less developed, and in which it is divided into two portions by a 

 furrow. 



In the unarticulated forms {Productus and Aulosteyes excepted^) the muscular im- 

 pressions appear more complicated, and by them Linyula, Crania, Discina, and others 

 may be distinguished ; but from the want of sufficiently well-preserved animals, the 

 function and character of all these have not been as yet completely or clearly made out, nor 

 can we add anything to what was published in 1802 and 1838 by Cuvier and Prof. Owen. 



In Discina for example, eight muscular impressions can be distinctly traced in each 

 valve ; the posterior pair in the perforated or ventral valve lie on either side of the inner 

 disk near the foramen, while the anterior two are situated fm-ther down towards the 

 centre of the shell. In the upper or dorsal valve the posterior scars are placed nearly 

 horizontally, at a short distance from the margin, and seem much smaller than the anterior 

 ones, which lie obliquely near the centre of the valve ; two other thin pair of diverging 

 muscles, destined to aflFect the shding of the one valve over the other, have likewise left, 

 impressions in the interior of the shell. 



1 In some species, such as T. spathulata and Davidsoniana (De Kon.), the hinge plate occupies 

 almost all the breadth of the hinge line, and is divided into four large concave spaces for the dorsal 

 peduncle muscles. 



2 These four pits are beautifully exhibited in the hinge plate of Spirigera eoncentrica. (See PI. VI, 

 fig. 66.) 



3 These two genera, although unprovided with teeth and sockets, possess a straight hinge line, and 

 present impressions of the adductor muscles essentially similar to those seen in all the other articulated genera; 

 the cardinal process never performs the function of a hinge-tooth, as was erroneously supposed by several 

 authors. 



