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



transverse sections, and the parosteal palatine and pterygoid are large 

 leaves of bone applied to the pterygopalatine bar ; the mesoptery- 

 goid is only half as large as them, but is relatively much larger than 

 in the adult. 



Whilst doubling its length, the young Lepidosteus gains a cranium 

 much more like that of the adult ; this is my fifth stage. The general 

 form is now intensely modified by the foregrowth of the rostrum : tho 

 " intertrabecula " is now two-thirds the length of the entire skull. The 

 cornua trabecular now reach only two-fifths of the distance to the end 

 of the beak, and the pterygopalatine arcade reaches but little further 

 forwards. Owing to the " tegmen cranii " being much larger, the 

 upper fontanelle is much smaller ; it is now a short oval, its longer 

 diameter being lengthwise. The bony matter of the " cephalostyle " is 

 now aggregated towards the hinder half of the notochord ; it is now 

 the basi-occipital bone. The exoccipitals and prootics are growing 

 larger, and there are now both sphenotics and alisphenoids. Also, 

 below, the quadrate, metapterygoid, and articular " centres " have 

 appeared ; and behind the jaw there are the hyomandibular, sym- 

 plectic, epi-hyal, cerato-hyal, and hypo-hyal centres ; and the epi-, 

 cerato-, and hypo-branchials have acquired a bony sheath. 



In a young Lepidosteus inches long (nearly), the approach to 

 the adult state of the skull has been very great ; the superficial bones 

 can all be determined. The most remarkable of these are the small 

 distal nasals and premaxillaries ; the long maxillary chain, ending in 

 an " os mystaceum and jugal " ; the extremely long and slender 

 " ethmo-nasals " and vomers; the small pre-opercular ; and the huge 

 angulated inter-opercular, which carries the large opercular and sub- 

 opercular. The five mandibular splints are all present (as in most 

 Sauropsida), the branchiostegals are only three in number, as in the 

 Carp tribe. 



The intertrabecula, which was merely a small tract of cells binding 

 the trabecule together, in front, is now three-fourths the length of the 

 entire skull; to it is due the length of the beak. The cornua 

 trabecular are now merely short lanceolate leafy growths on the sides 

 of the rostrum at its hind part. In the last stage there was a fine 

 bridge of cells running across behind the pituitary body ; this is now a 

 small cartilaginous " post-clinoid " bar. The opisthotic and epiotic 

 form now a scarcely divided tract of bone, all the other centres are 

 developing, and a pair of additional bones have appeared in the 

 funnel-shaped fore-end of the chondrocranium ; these are the " lateral 

 ethmoids." The bony matter of the basi-occipital has now retired to 

 the hinder third of the notochord, which has much shrunken. 



There are now two centres (as in Amia calvd) in the articular region 

 of the mandible ; the quadrate and metapterygoid centres are much 

 larger ; the hyo-mandibular and symplectic are together only half 



