72 LECTUllE IV. 



The Sand-lance (AmmoccEtes) presents a condition of the skull 

 Avhich corresponds with this first appearance of the cartilages in the 

 embryo of higher fishes (Jig. 24.). The occipital cartilages extend from 

 the sides of the pointed end of the chorda (ib. ch), and expand into 

 the acoustic caj)sules {ib. 16) : the sphenoidal arches {ib. 5), encompass 

 the pituitary or hypophysial space (/<y), now closed by a membrano- 

 cartilaginous plate, and unite anteriorly to form a small vomerine plate 

 {ib. 13), in front of which is the single undivided nasal capsule {ib. 19). 

 The now expanded cerebral end of the neural canal {Jig. 25. 7i) is 

 still defended by fibrous membrane only : but is divided from the 

 vomerine plate {ib. is), by a backward extension of the nasal sac 

 {ib. 19) to the pituitary vesicle. 



In the Myxine the acoustic capsules are approximated at the base 

 of the skull, near the end of the chorda : the sphenoidal arches are 

 longer, and unite with the palatine plate and arches, from which are 

 sent off the labial cartilaginous processes supporting the buccal ten- 

 tacles, answering to those in the Lancelet. In the long hypophysial 

 interspace of the sphenoidal arches a more or less firm cartilaginous 

 plate is developed, from which a slender median process is continued 

 forward to the vomerine or palatine plate, which supports the nasal 

 capsule ; another process extends backwards to the occipital cartilage. 

 Other processes are also sent off from the sides, which form a complex 

 system of j^eculiarly Myxinoid cartilages.* 



In the Lamprey {Petromyzon, Jig. 26.) the occipital cartilage is 

 continued backwards, in the form of two slender processes (c), upon 

 the under part of the chorda dorsalis {cli) into the cervical 

 region. The hypophysial space ijiy) in front of the oc- 

 cipital cartilage remains permanently open, but has been 

 converted into the posterior aperture of the naso-palatine 

 canal. I The sphenoidal arches (5) are very short, ap- 

 proximated towards the middle line ; and the presphenoid 

 and vomerine cartilage (l3) is brought back closer to 

 the sphenoidal arches. Two cartilaginous arches (24) 

 circumscribe elliptical spaces outside the presphenoid 

 plate : these apj^ear to represent the pterygoid ax'ches ; 

 but, as in the embryo of higher fishes, are not se- 

 parated from the base of the skull by distinct joints. The basal 

 cartilages, after forming the ear-capsules (l6) extend upwards upon 

 the sides of the cranium {Jig. 11.), arch over its back part, and leave 



* See Mliller's masterly Memoir, " Ueber die Myxinoiden," Abhandlung. der 

 Berlin. Akad. 1835, p. 105. tab. iii. 



■j- Agassiz (xxii.) describes this aperture as " un tres petit espece presque circu- 

 laire (e) dans laquelle est logee I'hypophyse du cerveau." The figure to which his 

 letter (e) refers is copied, like mine, from Miiller. 



