67] THE SKULL OF AMIURUS— KINDRED 67 



None of the suborbitals are sculptured. The third suborbital lies below the eye 

 and is nearer to the cutis than the other two. It Ues above the fascia of the 

 anterior fibres of the adductor mandibularis muscle. 



The two postorbitals are broader than any of the suborbitals. The first 

 curv^es around the posterior margin of the eye and is attached superiorly to the 

 inferior end of the second postorbital. The latter is the largest and longest 

 bone of the series and is slightly curved dorsally toward the anterior part of the 

 orbit. Ridges run on the anterior surface of the bone parallel to the course of 

 the enclosed canal. Both postorbitals are firmly embedded in the fascia of the 

 adductor mandibularis and the dilitator operculi muscles, some of their fibres 

 having their origin along the ventral surface of these bones. 



The most dorsal and posterior bone, the smallest of the series, is the post- 

 frontal. It lies dorsal to the superior end of the second postorbital, and like it, 

 is embedded in the muscle fascia. This bone is not as flat as the others, the 

 posterior margin being grooved, the margin of the groove projecting dorsally. 

 The posterior end of the suborbital canal, which passes through the other bones 

 of the chain lies within this groove. The anterior face of the bone is sculp- 

 tured. As remarked above, the postfrontal is connected with the frontal by 

 ligamentous tissue. 



The principal morphological feature of this series of bones is their relation 

 to the suborbital lateral line canal. From the lacrimal bone this canal extends 

 through the infraorbital chain into the frontal. As it passes from one bone to 

 the other it lies within the connective tissue which joins the two bones, and at 

 these points between the bones, from lacrimal to frontal, a dermal tubule 

 extends from the main canal to the surface of the integument where it opens by a 

 single pore. There are five of these dermal tubules and pores between the 

 posterior end of the lacrimal and the junction of the suborbital canal with the 

 supraorbital in the frontal bone. There is no tubule between the lacrimal and 

 the first suborbital nor between the postfrontal and the frontal. There is a 

 sense organ in each bone of the series (Fig. 11). 



The development of these bones has been studied by Klaatsch (1895) and 

 Schleip (1903). The former claims that all the bones develop from osteoblasts 

 which proliferate below each sense organ of the developing system. He main- 

 tains that all the bone connected with a canal arises from these same osteoblasts. 

 Schleip derives the lateral line containing element proper from these osteoblasts, 

 but goes no further. AUis (1898) thinks that true dermal elements are present, 

 comparable to those which give rise to such bones as the frontal, supraethmoid, 

 and other dermal bones of the head. In the development of Amiurus the bones 

 begin as Klaatsch and Schleip have stated, and in the 32 mm. stage the canals 

 are enclosed in tubular bones, all of which are close to the periphery of the 

 canal wall. The rest of the bone is developed from the ossification of the fascia 

 in the immediate region of these tubular bones, but I have not been able to trace 

 the derivation of the osteoblasts which cause the ossification. 



