758 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [12J 



The base of the occiput is occupied by the hasioccipital (Plate 1, 

 Figs. 2 and 3 ; Plate II, Figs. 4 and 5, Oh.). This bone has the form of 

 a mussel- shell, not unlike Cardium or Pecten. Posteriorly it is shaped 

 like the centrum of a vertebra, and presents for examination a tolerably 

 even and conical excavation, into which the anterior end of the chorda 

 enters. The margin of this excavation is connected by stout ligament- 

 ous bands to the centrum of the first vertebra, the anterior side of 

 which appears slightly convex. Articulating with the lateral margins 

 of the basiocciijital are the exoccipitals (Plate I, Figs. 1 and 3 01). 

 These two bones, for the greater part of the posterior aspect of the 

 primoidal cranium, assist in the formation of the lateral region only to 

 a small extent. In large specimens of Amia calva they join together 

 in the middle line over the medulla oblongata by means of a suture; in 

 immature specimens they are separated throughout their entire extent 

 by a strip of cartilage. They form no part of the articulation of the 

 neural arch of the first vertebra, but they are separated from it by two 

 bony arches, which rise upon the posterior portion of the hasioccipital, 

 having the form of a vertebral centrum, and which correspond in every 

 respect with the neural arch of the vertebra, and shall be termed the 

 occipital arches (Plate II, Figs. 4, 5, and 6, Oc. I and Oe.II).^^ 



The anterior occipital arch is formed by two triangular osseous plate- 

 lets, meeting together over the spinal cord, above which a non-paired ob- 

 long bone, directed upwards and backwards, is fastened by ligaments.^^ 



The posterior arch is similarly fashioned, only both of its parts are 

 of an oblong quadrangular shape, and develop on their posterior aspect 

 a smaU articular facet for the arch of the first vertebra. Upon this 

 arch is found also a pointed bone, directed upwards and backwards.^* 



The pointed bones resting upon the occipital arch are to be considered 

 as spinal processes. At the same time, however, I will remark that in- 

 asmuch as they are situated in a line with the uppermost interspinous 

 bones, which, indeed, no longer support the fins, one can just as well 

 count them in with the latter. The boundaries between the fin-rays and 

 the interspinous bones in Amia are not strictly defined, and the arrange- 

 ment or condition they present us with in this form furnishes another 

 proof that these formations originally had a genetic connection with 

 each other. A good drawing of these conditions has been furnished us 

 by Franque in Fig. 2 of his familiar treatise. 



The occipital arches of Amia are not of uncommon occurrence, hut are gen- 

 erally present either as independent arches, or reduced in various ways, or at- 



22 Reads ol»(/. in origmal text. — Trans. 



23 So I find the condition in the older specimens. In the younger individuals, from 

 which the illustration is taken, each half of the occipital arch consists of three sep- 

 arate osseous portions— -one lower -triangular piece, and two upper ones resting upon 

 it and situated behind one another. It is not possible to find an explanation for 

 this state of things at present. 



24 In the older specimens of ^mia the two pointed bones are blended into one osse- 

 ous plate. 



