240 PROCEEDINGS OF THE ACADEMY OF [1886. 



fully armed. All of them are very variable and very closely 

 related one to another. The species with the bony plates most 

 developed are most marine in their habit, while the partl}'^ or 

 wholly naked forms seem rather to inhabit the rivers. It may 

 be that a complete gradation exists from the fully armed aciileatus 

 to the naked williamsoni^ in which case all these forms should 

 be regarded as varieties of aculeatus. All northern regions seem 

 to possess full mailed species (aculeatus and vars.) and half 

 mailed species (allies of gymnurus). The former type seems 

 identical on both sides of the Atlantic, but the latter type so far 

 as our specimens can show seems to be different in America from 

 any described in Europe. 



Analysis of Species of Gasterosteus. 

 a. Sides entirely covered with (28 to 30) bony dermal plates. 

 Caudal peduncle keeled ; ventral spines each with a large 

 cusp on outer edge of base. Bones of head striate, the 

 skin ossified. Mouth large, little oblique ; maxillary not 

 reaching front of eye ; teeth small, in a single series in lower 

 and double series in upper jaw. The anterior plates are 

 joined above to the large bony plates at base of spines ; the 

 posterior plates are separated from the small plates at bases 

 of soft dorsal and anal by narrow naked strips. Lateral line 

 high up, parallel with outline of back and along middle of cau- 

 dal peduncle, which is strongly keeled. Dorsal spines serrate 

 on their edges. Origin of soft dorsal far in advance of origin 

 of anal. Caudal lunate ; anal spine recurved, smaller than 

 third dorsal spine, the fin similar to soft dorsal. Yentrals 

 long and slender, serrate. Pectorals narrow. D. I-I-I, 11 

 to 13; A. I, 9 or 10. 

 b. Body moderately robust, the depth 4^ in length. 



Aculeatus. 3. 

 bb. Body deeper and stronger ; caudal keel very strong. 



cataphractus. 3. (a.) 



aa. Sides partly covered with (2 to 15) bony dermal plates, the 



posterior part of the body naked. Ventral spines with or 



without cusp at base. 



c. Sides with 15 bony plates ; caudal peduncle keeled ; body 



slender; ventral spine long, almost or quite reaching vent ; 



dorsal spines in a straight line. Head 3^ in length to base 



of caudal ; depth 5. D. I-I-I, 11 ; A. 1, 8. Silvery below 



with dark bands across the body (Bean). Atkinsii. 4. 



