146 HOMOLOGIES. 



mouth a smooth eminence of cartilaginous density, tinted of a reddish-brown hue. This 

 terminates posteriorly in two brown chitinous rods, which diverge along the margin of a firm 

 valvular process (forming on each side part of the lips of the oral aperture) and support the axis 

 to which the " proboscis" is attached. A pointed and somewhat dense papilla lies at the fork of 

 the chitinous processes. On the dorsal aspect of the valves and in the central line of the animal a 

 series of transverse bars or ridges commences on each side. They are arranged in a double row, 

 separated by a well-defined median furrow, which corresponds with the groove between the dorsal 

 ridges externally. When first observed these firm bars had somewhat the appearance of a 

 vertebral column — split as in a dried fish, and this special chitinous skeleton might therefore furnish 

 the modern theorist with as good grounds as usual for the demonstration of the true stepping- 

 stone to the vertebrate series. They are upwards of seventy in number, commencing by a 

 well-marked chitinous bar just behind the fork of the axial processes, and, from the gradual 

 diminution of the rows, terminating in a somewhat pointed extremity. Generally the whole 

 structure may be separated into two divisions, viz. septal and branchial proper. Each septum 

 is furnished with a brownish chitinous rod, which is conspicuous throughout its entire length in 

 front, but is chiefly observed towards the median line posteriorly. These septa mark off 

 the branchial spaces, since by splitting and uniting with others at the outer extremity, a 

 branchial furrow is completed. Erom a point a little exterior to the median line each septal 

 rod passes outwards to bifurcate as already mentioned, its course being easily seen anteriorly on 

 account of the brownish hue of the process. This colour, however, is really confined to the 

 central part of the flattened organ, which has throughout a thin translucent edge above and 

 beneath, and is densest near the fork of the branchial lamella. Erom each side near its base 

 is given off a translucent lamina, which, with another from the adjoining septum, forms the 

 support of the branchial sabre. The junction of these laminae with the septal process is 

 interesting, for in transverse section the base presents the form of an anchor. The septal rod, 

 elevated on a fold of the basement- and mucous membrane, constitutes the strong central support 

 (shank) of the T-shaped structure, while the branchial laminae, passing from the transverse bar as 

 long recurved processes, correspond to the flukes. At the junction of the septal rod with the 

 transverse portion is a slight swelling of the former, having the brown chitinous part in the 

 centre, the rest of the process, as well as the branchial laminae, being quite translucent. As the 

 sections proceed outwards, however, a slightly brownish hue from the presence of dense chitinous 

 matter is seen at the base of the branchial laminae where they join the septal rod. The latter is 

 marked almost from the commencement by a vertical median line, showing its double composi- 

 tion. The branchial laminae at this part touch at the lower edges, but gape at the upper, so as 

 to make a triangular channel, which is completed by the thick membrane of the region. Eurther 

 outwards the branchial arches stand freely in their grooves, their supporting chitinous laminae 

 being enlarged at the upper end and bent inwards in transverse section, and the tunnel 

 completed by the membrane formerly described. The supporting chitinous rods gradually taper 

 from the median line to the outer edge, as also do those of the septal regions ; thus the diminu- 

 tion in the former case has to be compensated by an increase of the soft parts of the tunnel. 

 After the branchial lamella forms an independent sabre in the groove, the septal process is found 

 (in transverse section) elevated on a still higher fold of the mucous membrane as a club-shaped 

 structure, the central brown chitinous part — somewhat triangular in shape — appearing in the 

 rounded summit. The next change is the increase of the brown hue in the chitinous supports 



