III. COTTOIDS OF NORTH AMERICA. 23 



(24) is pointed forwards and triangular posteriorly. The dentary (32) is remark- 

 able for its broad lower branch entirely hollow ; the upper branch which bears the 

 teeth is scarcely more compact, and a little shorter. The teeth themselves are 

 exceedingly small and slender. The articular (29) participates in the broadness of 

 the dentary, is hollow like the latter, and provided with two spiny processes. The 

 angular (30) is a small scaly bone of little importance. The splenial has escaped 

 our notice; whether removed in preparing the skeleton, or entirely wanting, we are 

 not prepared to tell. To judge of it from its appearance in G. viscosus, it would 

 play no important part in the structure of this region. The liypotympanic (28) is 

 bifid on its posterior angle, and provided with a comparatively stout lower branch. 

 ThQ ijreiympanic (27) and mesotympanic (26), which fill up the space between the 

 concavity of the preopercular behind, the hypotympanic underneath, the ptery- 

 goids in front, and the epitympanic above, are almost membranous. Their shape 

 on the figure is rather ideal, as they had shrunk a few hours after preparation, so as. 

 to become shapeless. The epitympauk (25) is everywhere so characteristic as not 

 to be mistaken. It is provided with similar articulating heads or processes as in G. 

 viscosus, and connected with the same bones. 



The jjreopercidar (34) constitutes one of the characteristic features of this genus. 

 Its shape is as usual, crescentic; a broad channel passes through its centre, and the 

 posterior edge, or else the convexity of the crescent is furnished with four spines 

 or rather four acute processes, thinner, if possible, than the body of the bone itself. 

 The two superior spines are directed obliquely upwards, the uppermost is the 

 largest ; the two inferior ones, very minute, are directed downwards. The oper- 

 cular (35) is comparatively small, semi-membranous, uniform, and spineless. The 

 subopercular (36) and interopercular (37) are likewise semi-membranous, very small 

 and uniform ; their shape is accurately figured. 



The lateral line in T. Thompsonii (Fig. 11, I I) acquires a very great develop- 

 ment. When the skin is removed it appears like a uniform tube, cartilaginous in 

 its structure, exhibiting along the exposed sides, a series of very large holes, which 

 correspond to the external pores of the skin. It can be separated from the lateral 

 muscle in an unbroken chain [l. I.) from the tip of the caudal, where it terminates, 

 to the mastoid groove, where it is attached. Here it passes into the head and from 

 the mastoid groove a transversal supraoccipital canal establishes the first communi- 

 cation between the right and left. The suprascapular forms an arch or bridge 

 under which the lateral line proceeds forwards. When in front of the supra- 

 scapular the channel is covered by the scale-shaped snpramastoid (8'). A more 

 considerable branch runs into the preopercular, passing through the sup>ratym panic, 

 a tubuliform bone, which was not preserved on the specimens at our command. 

 From the preopercular, the channel passes into the articular, and thence into the 

 lower jaw, or dentary. In advance of the supratympanic tube, and horizontally, 

 we have the supjrcdadirymals. The lachrymal {lo) which limits the posterior edge 

 of the orbit is soldered to the posterior sulorhiial (73') thus forming a channel 

 which from the supralachrymal tubes, passes under the eye and into the iurhincd (19) 

 situated above the snout. From the supralachrymal tube an upper branch of the 

 channel passes into the postfrontals towards the orbito-frontal arcade, and through 



