SKELETON. GILL-CLEFTS. 251 



and consists in Ceratodus of a small dorsal piece, identified by 

 Huxley as the hyomandibular, and of a stout ossified ventral 

 piece which joins its fellow of the opposite side. A post -temporal 

 is present in Protopterus and Ceratodus but not in Lepidosiren. 

 There are two opercular bones, the operculum and inter- 

 operculum, each of which carries on its inner side a small band 

 of cartilage (cartilaginous opercular rays). The branchial 

 arches are five in number, slender, and often unsegmented. 



The pectoral girdle is placed just behind the head and consists 

 of a cartilaginous coraco-scapular arch, continuous with its 

 fellow ventrally. It is overlaid by a closely applied clavicle and 

 supraclavicle (cleithrum). The skeleton of the pectoral fin is 

 unibasal, rachiostichous and mesorachic. It consists in 

 Ceratodus (Fig. 138) of a segmented cartilaginous axis (axial 

 somactids) which tapers distally and proximally is attached to 

 a stout basal somactid. The latter is articulated to the shoulder 

 girdle. Attached to the pieces of the axis are on each side a 

 number of small segmented somactids which carry a fringe of 

 dermotrichia. The fin is therefore fringed on the crossopterygian 

 type and its skeleton constitutes what has been called the 

 " archipterygium." The pelvic girdle is a single piece of cartilage 

 in the middle line, with a forwardly directed process. The 

 pelvic fin skeleton is very similar to the pectoral. In Lepidosiren 

 the paired fins are filamentous and the skeleton consists of the 

 axis of Ceratodus only, without the lateral somactids and dermo- 

 trichia. In Protopterus somactids and dermotrichia are present, 

 though in a reduced form. 



Gill-Clefts. There is no spiracle ; the hyoid arch bears a 



gill which in Ceratodus is a pseudobranch supplied by arterial 



blood, but in Protopterus is a gill supplied by a branch from the 



afferent vessel to the first branchial arch. In Ceratodus and 



Protopterus there are five gill-clefts, the last being behind the 



4th branchial arch. In Lepidosiren the hyobranchial cleft is 



closed in the adult though open in the larva. The partitions 



between the gill-clefts are membranous septa to which the 



branchial lamellae are attached, thus approximating to the 



condition found in Selacliians. In Ceratodus the four anterior 



branchial arches bear a double row of lamellae ; in Protopterus 



and Lepidosiren* the 1st and 2nd branchials are without gills, 



* There is considerable individual variation in the respiratory 

 lamellae of Lepidosiren. 



