The oriil cirri of Siluroids and the origin of the head iii Vertebrates. 389 



block. The posterior portion has undergone partial resorption and 

 the posterior angle is far from reaching the quadrate. The upper 

 angle is i)rolonged into a vertical somewhat anteriorly directed coro- 

 noid process {Proc. cor.) to which is affixed the procartilaginous coro- 

 noid piece (Cor. p.) which is large and passes coutinously into the 

 coronoid tentacle. The tentacle (Cor.t.) turns downwards outwards 

 and backwarils. The anterior angle of the Meckeliau cartilage is 

 I)roduced into the meutomeckelian process (m.Mck.) which is short 

 and very far from reaching its fellow of the opposite side. The 

 distance of the mentomeckelian processes from each other is well 

 shown in the figure. 



A stylohyal cartilage (Sti/.hi/.) is present and bears the cerato- 

 hyal which is very thick at its two ends. In the specimen the middle 

 portion was replaced by perichondrial bone on one side. The distal 

 extremity bears a hypohyal. 



The postero-ventral tip of the ceratohyal bears a procartilaginous 

 prolongation (* Fig. 9) which supports the uppermost branchiostegal 

 ray. It may perhaps be considered the homologue of one of the 

 cartilaginous branchiostegal rays of Selachii. 



3Io(lel of Calliehthys (Figs. 6, 7, 10). 



Specimen 3 cm in length. Sections 25 (.i thick. Every second 

 section drawn, with magnirtcation 28. Zeiss, Oc. 2, Obj. aa, height 

 of drawing above table 19 cm. Wax plates 1,2 mm thick. 



Only the end of the snout reconstructed. The specimen was young 

 but replacement of cartilage by bone has proceeded to a considerable 

 extent, especially in the rostral region. 



The median cartilage of the rostral region slopes sharply down- 

 wards and forwards, and, viewed from the side, its profile is curved. 

 The floor of the nasal cavity extends laterally as two wings, actually 

 prolonged still more laterally by the prefrontal bone. At the median 

 anterior part of the rostral region there are abundant traces of re- 

 sorption of cartilage, showing that a more distinct rostral prolongation 

 was present at an earlier stage. This resorption is the cause of the 

 irregidar appearance of the anterior face as seen in the model. 



Below the junction of the lateral and medial portions, a remark- 

 able though slight projection of cartilage (* FTg. 7) bears the pre- 

 l)alatine bones, of which the large cartilaginous apophyses are seen 

 in the figure, placed curiously close together and , along with the so 

 called ethmoid bone, reaching very near to the tip of the snout. 



