III. COTTOIDS OP NOKTH AMEKICA. 21 



backwards. The lower and anterior extremity of the preopercular is terminated 

 by an acute spine. The upper angle of the opercular terminates by a stout spine 

 similar to that of the preopercular, although less powerful. The subopercular is 

 provided with a slender and acute spine, directed upwards along the anterior edge 

 of the opercular. Finally, the posterior extremity of the interopercular is very 

 acute. 



On the other hand, the suborbitals do not present any striking difference. The 

 upper branch of the dentary is a little shorter than the lower one ; the mucous 

 channel of this bone is quite prominent. The horizontal branch of the premaxillary 

 is a little longer than the vertical one. The hypotympanic is provided along its 

 lower edge with a stout, acute and slightly curved spine, grooved underneath for 

 the reception of the lower branch of the preopercular. The mesotympanic is 

 entirely ossified, lying nearly horizontally above the hypotympanic spine. 



The scapular arch and pectoral fins are similarly constructed in Cottus and 

 Acanthocottus. 



The position of the ventral fins is identical. The pubic bones alone differ some- 

 what in shape, being almost three-winged in Acanthocottus. 



The vertebral column of A. virginianus is composed of thirty-six vertebrae, five 

 more than in C. viscosus. Of this number twelve are abdominal, nine thoracic, 

 and three pelvic, each bearing a pair of ribs ; the pelvic vertebrae being provided 

 as in C. viscosus with styliform pelvic bones directed obliquely backwards, the 

 extremities of which are fixed to the haemapophysal arch of the caudal vertebraB 

 by means of ligaments. The caudal vertebrae are twenty-four in number, four 

 more than in G. viscosus. The structure of the vertebrae themselves is very 

 similar. In the first place the insertion of the ribs takes place in the same way, 

 the anterior pair on the neural arch (Fig. 30), the posterior ones on the haemal 

 arch (Fig. 31), whilst the intermediate pairs are attached on the body itself of the 

 vertebrae, at different heights. The pelvic appendages are not represented on Fig. 

 31 ; their position is the same as in C. viscosus (Fig. 5 d *), immediately under 

 the ribs. The sole difference is, that in A. virginianus the pelvic vertebrae have a 

 complete haemal arch, so that all the pelvic bones are inserted on it, whilst in G. 

 viscosus the first pair is nearer the centrum, the haemal arch of the ninth vertebrae 

 being incomplete. Fig. 30 exhibits rudiments of parapophyses. A caudal 

 vertebra is represented in Fig. 32, in order to show the neural canal somewhat 

 different from that of C. viscosus. The neural arch and spine are more erect 

 (Fig. 30), and the neural canal higher than broad; whilst in the caudal vertebrae 

 (Fig. 32), this is broader than high, exactly the reverse of its structure in G. viscosus. 



5. THE BONY FRAME OF TlUGLOPSIS THOMPSON!!. 

 PLATE II. Fig. 11 ; and PLATE III. Fig. 2225. 



The external delicate appearance of our fish is a direct reflex of its internal 

 frame, which is composed of pieces of an extreme delicacy. 



With reference to the general figure of the skeleton (Plate II., Fig. 11), we 



