792 EEPOET OF COMMISSIONEE OF FISH AND FISHERIES. [46j 



Of all tliese plates on the superior aspect of tlie skull thefrontals are 

 by far the largest. Posteriorly they articulate with the squamosals 

 and parietals as already described, while on either side tbey make room 

 for the postorbitals. (Plate IV, Fig. 16, Fr.) 



Their anterior bodies are separated from the hinder margins of the 

 nasals by a considerable interspace. This is bridged over by a delicate 

 membrane, which is continuous with a similar tissue that extends across 

 the gap between the frontal and lacrymal on either side (Plate IV, 

 rig. 16, jM.). In prepared skulls where this structure is allowed to 

 remain and dry it becomes very thin, and by cutting through it we ex- 

 pose the posterior narial apertures and the primoidal cranium beneath. 



The nasals are oval bones that articulate with each other in the 

 median line by means of a markedly dentate suture. Wedged in between 

 t|iem anteriorly we find the azygos and subtriangular ethmoid (Plate 

 IV, Fig. 16, Wa. and Eth.). Upon the outer side of each nasal, in my 

 specimen, there lies a smaller plate, of a spindle-like form, that corre- 

 sponds to the plate described by Bridge as the preorbital, although its 

 posterior end occupies a point only about half-way distant between the 

 teeth and the anterior margin of the orbit (Fig. 16, An.). This author 

 also figures a small ossification below this preorbital, which does not 

 occur in my specimen. Dr. Sagemehl seems to have found a like 

 structure, but attached no significance to it. 



Bridge describes the ethmoid very concisely when he says, " The 

 dermo-ethmoid {Eth.) is somewhat T-shaped, with its anterior trans- 

 verse part slightly concave from side to side. It overlies the prenasa' 

 process and the prem axillae. Each end of the transverse part is in 

 contact with the i)reorbital bone, while the stem of the T passes back- 

 wards between the nasals, separating them for about a third of their 

 extent." ^^ 



The periphery of the orbit is subelliptical in outline, and six of the 

 dermo plates contribute to its boundary. The upper half of the cir- 

 cumference is formed by the free margin of the frontal, as the vault of 

 the orbital cavity is made by this bone. Its lower half is bounded by 

 the five remaining plates, of which the superior postorbital is the largest, 

 and the rear suborbital the smallest, though the latter contributes the 

 greatest share to the peripheral circumference. 



The most anterior bone of this suborbital chain, I call, in common 

 with other anatomists, the lacrymal, as it is quite constant in the class, 

 both in the position it usually occupies and its occurrence. The two 

 smaller plates, immediately beneath the orbit, ai-e true suborbitals, and 

 their number and arrangement vary greatly throughout all fishes. 



Behind the large triangular postorbitals, we find a group of small 

 bone-plates, forming a vertical chain, that fills in the space between 

 these bones andpreoperculum (Plate IV, Fig. 16 7;;, ¥, h"). These small 

 plates seem to vary in their size, form, and number, for on the opposite 



59 Jonr. of Anatomy, July 1877, page 608. 



