1881.] Development of the Skull in Lepidosteus osseous. 109 



Behind, the parachordals have grown further along the thick noto- 

 chord, and on each side they are now confluent with the auditory 

 capsules, which have become irregularly ovoidal through the growth 

 of the large semicircular canals within ; their basal fenestra is still a 

 round space under the partially floored " sacculus." The trabecule 

 swell out where they are confluent, and then are narrower in front 

 again. At their fore end each band passes insensibly into the corre- 

 sponding palato- quadrate bar outside, whilst inside they are separated 

 by a large pyriform wedge of cartilage, the intertrabecula. The 

 thick, rounded, free fore end of this median cartilage is the rudiment 

 of the great "nasal rostrum," and the rounded fore ends of the 

 trabecule are the rudiments of their " cornua." 



There is only a floor in the occipital region, but the wall-plate of 

 the chondro cranium has begun as a styloid cartilage running forward 

 from the fore end of each auditory capsule into the superorbital region. 

 The palato-pterygoid band — continuous in front with the trabecule — 

 is now longer than the proximal part of the suspensorium, the spatulate 

 quadrate region whose dorsal end is the free " pedicle." The wide 

 proximal part of each trabecula is now already forming an oblong- 

 facet, the " basi-pterygoid," for articulation with the' facet of the 

 " pedicle." 



An oblong concavity is now seen under the outer edge of each 

 auditory capsule, for the oblong head of the hyomandibular whose 

 body is still solid, and the " symplectic " part merely a process growing 

 downwards and forwards to get inside the lower margin of the sus- 

 pensorium of the mandible. The epi-hyal region is still merely the 

 top of the cerato-hyal ; it is afterwards separate as a bony centre ; the 

 inter-hya], hypo-hyal, and double basi-hyal are all now chondrified and 

 distinct, but the branchial arches are not yet segmented. In this 

 stage the skull is a curious compromise between that of a Salmon at 

 the same stage and that of a Tadpole just beginning its transformation. 

 The hind-skull is quite like that of a young Salmon, the fore-skull, 

 with its non-segmented palato-quadrate, and its forwardly placed 

 quadrate condyles and horizontal suspensorium, is very much like 

 what is seen in the suctorial skull of the Anurous larva ; a splint bone, 

 the parasphenoid, as in the Tadpole, has now made its appearance. 



The largest embryos reared by Messrs. Agassiz and Garman, which 

 are about one inch in length, form my fourth stage ; these are rapidly 

 acquiring the characters of the adult. 



This is the stage in which the chondrocranium of this Holostean 

 type corresponds most closely with that of the Chondrostean Sturgeon, 

 whose adult skull is similar to that of Garpike just as the latter begins 

 to show its own special characters. This important difference is already 

 evident, namely, that whilst in Aeipenser the olfactory capsules remain 

 in the antorbital position, those of Lepidosteus are already carried 



