MOKPHOLOGY OF CRAXIAL iMUSCLES IN SOME VERTEBRATES. 185 



iindei'goes various changes in the specimens examined. In 

 Acipenser embryos up to the length of 11 mm. it remains 

 undivided, hut in the adult (Vetter) it has additionally spread 

 up to the skull. InSaltno it additionally spreads backwards 

 to thehyomandibula. In Lepidosteus, Amia (Text-figs. 28, 34), 

 and probably (vide infra) Polypterus, the adductor mandi- 

 bulee divides into internal and external portions. In Lepi- 

 dosteus (Text-figs. 26, 27) the internal portion extends upwards 

 outside the palato-quadrate to gain an additional origin from 

 the cranial wall; whilst the external portion, keeping its 

 original origin, arises from the external surface and upper 

 end of the quadrate outside the insertion of the levator arcus 

 palatini. In Amia^ the internal portion (in 11 mm. embryos) 

 sends forwards a projection from its upper end which forms 

 the muscle connected with the olfactory chamber (LAPj of 

 McMurrich, L^IS^^ of Allis); in 14' mm. embryos the re- 

 mainder of the internal portion extends upwards above the 

 level of the palato-quadrate and divides into three parts 

 (LAPo, 3, 4 of McMurrich, LMSo, i, ^ of Allis). The external 

 portion of the adductor additionally extends backwards to 

 the hyomandibula in 9 mm. embryos, and divides into the 

 parts described, as parts of the adductor, by Allis. 



In Polypterus, Pollard described the adductor as consisting 

 of three parts, the pterygoid, temporal, and masseter- ; of 



' The adult condition of the mandibular muscles of Amia has been 

 described by McMurrich and hj Allis. McMurrich stated that they 

 consist of a levator arciis palatini and an adductor mandibulse, and 

 that the former is divided into five parts — from behind forwards 

 LAPj, o, 3, 4, 5 ; of these LAP, is inserted into the metapterygoid with 

 some of its hindmost fibres into the operculum, LAP^, 3, ^ join the 

 adductor, and LAP- is in connection with the olfactory chamber. 

 Allis separated LAP, into a dilatator operculi and a levator arcus 

 palatini, whilst LAPo. 3. 4. 5 he called the second, first, third and fourth 

 divisions of the levator maxillse superioris. He suggested that LAP ^ and 5 

 (LMS3 and J are derived fi-om add. j3 of Selachians, and that (his) 

 levator arcus palatini and dilatator operculi are the homologue of add. 

 7 of Selachians. From the embryological findings mentioned above it 

 would appear that a new nomenclature for LAP.,. 3, ^. - (== LMS^, ,, 3, 4) 

 is needed — in terms of an internal adductor. 



2 Pollard thought that the pterygoid and temjjoral were the homologue 



