1936] Schults: Keys to Fishes 205 



Infraoral. Below the mouth. The teeth of the mouth or disk be- 

 low the oral opening, in lampreys. 



Infraorbitals. A chain of small bones below the eye. 



Insertion of fin. A term applied to the point where the paired fins 

 begin or arise from the body. 



Interhacmal spines. Elements supporting the anal fin rays. 



hiterhaemals. See interhaemal spines. 



Interhyal. Upper hyoid bone attached to hyomandibular. 



Inter maxillaries . The premaxillaries. 



Interneural spines. Elements supporting the dorsal fin rays. 



Interneurals. See interneural spines. 



Interopercle. Membrane bone between the preopercle and the 

 branchiostegals, usually anterior to subopercle when latter is 

 present. 



Interorbital space. The distance between the eyes on the top of the 

 head. The bony interorbital space is measured unless other- 

 wise specified. 



Interspinous bones. The interneurals and interhaemals. 



Isocercal. Said of the tail of fishes when the last vertebrae progres- 

 sively become smaller and smaller and end in the median line 

 of the caudal fin, the hypural plate being nearly obsolete. 



Isthmus. The region just anterior to the breast of a fish where the 

 gill membranes converge. The fleshy interspace between gill 

 openings. 



Jugular. Pertaining to the lower throat; said of the ventral fins 

 when placed in advance of the attachment of the pectorals. 



Keeled. Having a ridge along the middle line. 



Lacustrine. Living in lakes. 



Lamellae. Plate-like processes like those inside the bill of a duck. 



Larva. An immature form, which must undergo change of appear- 

 ance before becoming adult. 



Lateral. Pertaining to the side. 



Lateral line. A series of sensory tubes opening to the exterior or a 

 sensory canal along the sides of a fish, sometimes single, some- 

 times multiple. 



Lateral line with an arch in front. The lateral line has a distinct ele- 

 vation over the pectoral fin in the form of an abrupt arch, 

 not a mere curve. 



Lateral line with an accessory dorsal branch. An extra dorsal branch 

 begins on the head and runs posteriorly off the main lateral 

 line. 



Lateral processes. See parapophyses. 



