54 INTRODUCTION. 



impressions at times divided by a dorsal mesial plate ; and a glance at the figures introduced 

 in Plates VI, VII, VIII, and IX, will explain their position and modifications in the 

 difierent genera better than could be done by a lengthened description. 



The muscular fibres that produce this quadruple impression, after converging to near 

 the centre of the ventral or dental valve, are there fixed, and produce a single impression (at 

 times divided) ; but in some species of Spirifer, such as S. rostrafus, Munsteri, &c., where 

 there exists a largely developed ventral septem, the base of which occupies the whole space 

 generally devoted to that muscle, its attachment is then shifted to the upper sides of 

 the plate, as represented in PI. VI, fig. 60. 



which I employed, and which, as our plates are printed, it is now impossible to harmonize except by the 

 following comparison here added to assist the reader : 



Prof. Owen. Davidson. 



Adductor longus anticus . 



Adductor rnvsclt 



]- 



Adductor longus posticus 



Adductor brevis = Cardinal muscle. 



Cardinalis = Accessory cardinal. 



Retractor superior ") C Dorsal. 



Pedicle muscles 



]- 



Retractor inferiori^of the Peduncle) ) (_ Ventral. 



From the above, it will be seen that I used Professor King's term cardinal muscles for Professor Owen'g 

 adductor brevis and cardinalis. The terra pedicle muscles (which I divide into dorsal and ventral) appeared 

 to be a name which it was very desirable to retain, and I find that the distinguished Hunterian Professor 

 admits, in p. 8, that his retractor superior and inferior are esseniiaWy peduncle muscles. 



In the 'Ann. and Mag. of Nat. Hist.' for July, 1846, Professor King enters into the following details 

 relative to the muscular system oi Terebratula, "in order that the use of certain parts to be mentioned 

 hereafter may be properly understood. From a specimen of T. dorsata, at present before me, containing 

 the entire muscular system desiccated, and freed of the visceral mass, I have drawn up the following 

 details : — The rostral or urabonal cavity is occupied with a dense fibrous cylindrical body called the pedicle ; 

 considering the convexity of the foraminal valve, as the upper side of the shell, the inferior end of the 

 pedicle fits into the foramen ; while its superior end, which is somewhat flattened or dilated in the trans- 

 verse direction of the shell, is situated at the entrance or anterior part of the rostral cavity, to the surface 

 of which it appears to be attached by means of tendinous or membranous cords : the truncated extremity 

 of the pedicle itself not being adherent. A little in advance of the upper extremity of the peduncle, three 

 pairs of muscles pass off to different parts, the outermost pair (which consist of those muscles implanted 

 neai'est the lateral margins of the valve), passes at a slight angle into the upper part of the pedicle : within 

 these muscles, and somewhat in front of them, another pair passes downwards (slightly converging at th^ 

 same time), and becomes attached to a flattened prominency, situated in the centre of the hinge of the 

 lower or imperforate valve. To distinguish these pairs of muscles from each other, it will be necessary to 

 name the former the superior pedicle muscles, and the latter the cardinal muscles. In close proximity to 

 the superior end of the pedicle, and a little behind, and within the cardinal muscles, and therefore near the 

 medio-longitudinal line of the shell, is situated the origin of the remaining pair, which passes directly 

 down to a little behind the centre of the opposite valve, each muscle at the same time becoming dichoto- 

 mous in its inferior half; these may be termed the valvular muscles. Besides supporting the cardinals and 

 the valvulars, the imperforate valve affords attachment to other two muscles which pass upwards from the 

 crural base (where each one is divided), and become inserted in the upper part of the pedicle : it is proposed 

 to name these, the inferior pedicle muscles. With one exception, the foregoing description agrees with that 



