252 



ZOOLOGY 



SECT. 



or two separate apertures on the summit of a urinogenital papilla 

 into the cloaca at the base of the cloacal csecum referred to 

 above. Many of the cellular elements, such as the blood-corpuscles, 

 are of comparatively large size. There is holoblastic, but unequal, 

 segmentation, as in Ceratodus, followed by a true invagination. A 

 pair of medullary folds are developed, and between them is formed 

 a median solid ectodermal keel in which a neuroccele only appears 

 subsequently. The larva has well-developed external gills. 



The Dipnoi are a very ancient race. The genus Ceratodus itself 



FIG. 925. Frotopterus. Skull, shoulder-girdle, and skeleton of fore-limb. AA\ splenial : AF, 

 pre-orbital process ; a and b (on lower jaw), and S.L, teeth ; b, basal cartilage of pectoral 

 fin ; B, ligamentous band connecting the mandible with the hyoid ; co. ligamentous band 

 connecting the dorsal end of the pectoral arch with the skull ; D, angular ; FP, fronto- 

 parietal ; Ht, membranous fenestra perforated by the foramen for the optic nerve (//); 

 By. hyoid ; K, external gills ; Kn, Kn l , cartilage of the pectoral arch ; KR, occipital rib ; 

 LK and MK, investing bones of the pectoral arch ; NK, olfactory capsule ; Ob, auditory 

 capsule ; Occ. supra-occipital ; Op. and Op 1 , rudimentary opercular bones ; PQ. palato- 

 quadrate ; Psp. Psp 1 . spinous processes of the anterior vertebra? ; SE. supra-ethmoid bone , 

 SK, roofing investing bones ; Tr. palatoquadrate cartilage ; WW 1 , anterior vertebra 

 coalescent with the skull ; I-V, branchial arches (that marked / is forked and the anterior 

 bar may represent the first, in which case there are six branchial arches) ; 1, 2, 3, segments 

 of axis of pectoral fin ; *, *, vestigial lateral rays of pectoral fin. (From Wiedersheim.) 



extends back to the early Mesozoic, and the remains of allied forms 

 (Dipterus and other genera) are found in Devonian and Carboniferous 

 formations. But if, as is conjectured, the Arthrodira are to be 

 regarded as Dipnoi, then the group dates back as far as the Silurian. 

 The evidence for this conclusion is, however, by no means complete, 

 as our knowledge of the structure of the extinct Fishes in question 

 is necessarily meagre. They had the head and anterior part of 

 both dorsal and ventral surfaces (Fig. 926) protected by bony plates, 

 the system of head-plates being connected with those on the trunk 

 by a well-developed movable joint. The notochord was persistent, 

 with partly calcined neural and haemal arches, and the cranium was 



