710 MR FRANK J. COLE 



front, the fibres of the muscle are attached to the free margin and the dorso-internal 

 surface of each limb of the posterior segment, but behind, as the latter loses its 

 U -shape and becomes flattened dorsally, and the muscle extends dorsally and laterally 

 to it, the attachment spreads over the whole dorsal surface of the cartilage, and the two 

 halves of the muscle therefore meet at the mid-dorsal line of the posterior segment of 

 the basal plate. Hence we cannot say that the cartilage is everywhere interpolated in 

 the course of the muscle fibres, and thus represents a modified portion of the muscle, 

 since, anatomically, there is nothing to show that the muscle was ever more extensive 

 than it is now. However, the connection between the cartilage and the muscle is of 

 so intimate a nature that the former has been stated by Ayers and Jackson to 

 represent the tendon of the latter which has become converted into a simple form of 

 cartilage, and which was originally inserted into the posterior extremity of the middle 

 segment of the basal plate. To this view I assented in my first Part (p. 756), but on 

 further consideration it seems impossible that the muscle in question can be the 

 copulo-copularis, since this is a transverse constrictor muscle, and as such has no 

 tendon at all. Schaffer, in two earlier papers, regarded the posterior segment as a 

 sesamoidal formation in the tendon of the " M. retractor linguae," but even if this is 

 not a slip for copulo-copularis it cannot be correct. We must, however, here note 

 that the posterior region of the copulo-copularis is U-shaped in transverse section, 

 and in this respect is similar to the cartilage, but the conversion of the anterior region 

 of the muscle into the cartilage postulates a series of changes of which there is at 

 present no evidence whatever. 



Dorsally, the two halves of the copulo-copularis are separated by a median 

 ligamentous linea, which in front is a specialisation of the stout fibrous roof of the 

 posterior segment of the basal plate. This linea is at first narrow, but gradually 

 widens as it passes backwards. At the middle of the muscle the two halves begin to 

 diverge dorsally, and this divergence continues up to the posterior extremity, where 

 the two halves were 6 mm. apart in the above specimen, and the longitudinalis lingua? 

 was plainly visible between them. They are, however, connected by a tough sheet of 

 transparent fascia, which in front is directly continuous with the opaque linea above. 

 The lateral margin of the linea and of the sheet of fascia throughout the greater part 

 of the thickest portion of the muscle sends down on each side a vertical longitudinal 

 fibrous sheet, which extends in places through the thickness of the upper half of the 

 muscle, forming an intra-muscular fascia inclining towards the mid-dorsal line. Fibres 

 are attached to both sides of this sheet, so that in transverse section it has a bi- 

 pinnate appearance. 



Ventrally there is also a median linea, which is wide in front before the two halves 

 of the muscle meet at the middle line, but behind it is very narrow from side to side, 

 although traversing the thickness of the muscle vertically so as to completely separate 

 the two halves below. At the posterior extremity the inferior chondroidal bar plunges 

 into the muscle and gives origin to a few of the fibres. In Bdellostoma, J. Muller 



