178 Charles Seariuar Mead 



stniction of another j^ig's head, the drawings of which I have had 

 access to. In this head it cuts clear through the lamina, dividing 

 it into two parts and uniting with the fenestra spheno-parietalis. 

 Spondli (1846) and Kolliker (1879) have indicated it plainly in 

 their drawings of the pig embryo, where it lies just in front of 

 their "squama occipitalis." In Decker's drawings (1883) of a pig 

 embryo much older than mine, he has placed this fissure further back, 

 and with the apex directed toward the posterior part of the ear- 

 capsules. According to his conception, this fissure separates the 

 lamina parietalis (cartilago parietalis) from the tectum posterius 

 (squama occipitalis). In his drawings of the sheep embryo, the fis- 

 sure is further forward and points in the same direction as it does 

 in his Sus drawings. In Bos, according to his figure, it is absent. 

 How far it is present in the other mammals, I am not able to say. 

 It is probable that it occurs in most mammals during the formation 

 of the lamina parietalis. 



Anteriorly the lamina parietalis passes, without demarcation, into 

 the broad commissura orhito-parietalis and thence into the ala orhi- 

 talis. The lamina parietalis with the commissura orbito-parietalis 

 together constitute the homolog of the narrow reptilian taenia mar- 

 ginalis (Gaupp, 1900). This broad, plate-like condition may be 

 secondarily developed in the mammals, or it may be an inheritance 

 from their prereptilian ancestors; that is, it may have come from 

 the solid side-wall of the amphibian skull. From his studies on the 

 development of the skull of Echidna, Gaupp considers the broad 

 plate-like form of this commissure a primitive condition. 



Posteriorly the dorsal part of the lamina ends freely, while that 

 below the fissura laminae parietalis is continued into the tectum 

 posterius. Ventrally it is connected with the anterior and posterior 

 comers of the vestibular portions of the ear-capsules, while between 

 these two connectives lies the large crescentic foramen jugulare 

 spurium. In some forms (cf. Talpa) this serves for the passage of 

 large veins going from the sinus transversus to the vena jugularis 

 externa, but in the specimen of Sus from which my reconstruction 

 was made only a very small vein passes through this foramen. The 

 remainder of the foramen is filled with connective tissue. A similar 



