70 MR. W. K. PARKER ON THE STRUCTURE AND 



the Lamprey, but it has already gone beyond the suctorial type in having both paro- 

 steal and ectosteal plates ; it is profitable also for comparison with that of the larvte of 

 Dactylethra and Pipa. 



The free growth of cartilage, here, gives us a character in this type which I have 

 not found in others, but which Gotte describes in Bomhinator, namely, that the noto- 

 chord (nc.) is ensheathed in an azygous, tubular cartilage, as in the Salachians.'"' 



Hence the parachordals are very narrow bands between this tube and the ear-sacs ; 

 and veiy short also ; for the trabecular apices embrace the fore end of the notochord. 



The cartilage, right and left, has formed a perfect ai'ch above, and the flat roof runs 

 up to the post-orbital region (Plate 2, figs. 1 and 3, f>.o.). 



Behind (Plate 2, fig. 3), the occipital condyles are forming ; they look directly back- 

 wards, and the bony matter (e.o.) runs into them. The auditory capsules wei'e not 

 relatively large at first, but they are so largely developed outwards now as to seem 

 of unusual size, being winged above and below. 



By the study of the growth of the skull in many types I have satisfied myself that 

 the upper wing (" tegmen tympani ") has not the same morphological import as the 

 lower, or floor of the tympanium, both of which are enormously developed. 



The basal plate, formed by the trabeculte and parachordals, gro\\s round the base of 

 the ovoidal ear-sacs, and appears outside, especially in front : hence this copious growth 

 of cartilage, forming elegant loiver wings to the ear-masses, and serving as a shelving 

 tympanic floor {f.ty.). This undergrowth showing itself outside the ear-sacs, appears 

 in many forms ; it is a manifest striving of the basal plate to meet the arches that 

 belong to it, this junction being impeded by the huge sense-capsules impacted in at 

 this part. 



But, as in the " Aglossa," the "tegmen tympani" {t.tij.) is a superficial growth, of 

 cartilage appljnng itself to the capsule, in the margin of which is imbedded the hori- 

 zontal canal ; this roof-plate is very large, still larger than the floor (Plate 2, figs. 1-3). 



This tegmen is very thick o,s well as wide ; it is widest in front, and there it over- 

 laps, and is confluent with, a large process from the elbow of the suspensorium (o^.p.). 



Here the tegmen cranii, and the tegmen tympani, of each side, remain soft, but the 

 labyrinth is almost entirely enclosed in a generalised occipito-petrosal mass {e.o.,pr.o.). 



This is an undistinguishing spi'ead of bony matter over and under these regions, a 

 division into " joeriotic " and " ex-occipital " being attempted later in the growth of 

 the head. 



It is very solid bone, and leaving the azygous cartilage untouched below, and a 

 similar breadth of roof above, it builds a side wall to the foramen magnmn, forming a 

 solid I'ough kind of masonry, and enclosing most of the "canals" and part of the 

 " sacculus." 



The cartilaginous stapes (Plate 2, fig. 3 ; Plate 10, fig. 5, st.) lies in its fenestra just 

 under the middle of the "tegmen," and above the deep shelving floor (f.ty.) 

 * The outer shcatli of the notochord is extremely thin iu most of the Batracbia. 



