GA S TEROS TE I FORMES 



4ii 



the eye, and the mouth becomes terminal (Figs. 410-13). The 

 maxillae are toothless, and almost or quite excluded from the margin 

 of the gape. The branchial apparatus becomes reduced. The 

 cranial bones remain superficial ; the frontals joining the supra- 

 occipital, and separating the parietals when these are present 

 (M'Murrich [289]). Ectopterygoids are generally absent. A short 

 eye-muscle canal may be present, but it is usually vestigial or 

 absent. The post-temporal is never typically forked, and is usually 

 fixed to the skull. The supra- and postclavicle are generally absent. 

 A foramen is present in the scapula, or between it and the cleithrum. 



The paired fins are often reduced, especially the pelvics, which 

 are abdominal in position. Typical scales are rarely present, being 

 generally represented by rows of scutes of an osseous substance 

 without bone -cells. These scutes 

 may be ankylosed to the endo- 

 skeleton, and may form a complete 

 armour in Lophobranchs. Large 

 paired scutes come into secondary 

 connection with the limb -girdles, 

 and are almost always present, 

 firmly fused in the adult to the 

 ventral edge of the coracoids (Figs. 

 406, 407). It is these bony plates 

 which were formerly supposed to 

 represent ' infraclavicles ' (true 

 clavicles), homologous with those 

 of the Chonclrostei and Osteolepi- 

 doti. Spines are sometimes pre- 

 sent in the fins (Gasterosteoidei). 

 The canal of the lateral line, the 

 mesentery, and the pyloric caeca 

 are lost as a rule. The gills are 

 reduced to four or less. 



Most of the families of the 

 Gasterosteiformes can be traced back 

 to Eocene deposits. The origin of 

 the sub-order is still quite unknown. 



Tribe 1. Gasterosteoidei (Hemi- 

 branchii, in part). 



the least 



Ventral view of the trunk of Gasterosteus 

 spinachia, L. a, anus; a.f, spine of anal 

 (in; b.ui, confluent branchiostegal mem- 

 branes; dc, dermal plate attached to pec- 

 toral girdle ; l.pl, lateral dermal plates ; 07), 

 The skull operculum ; j>, dermal plate attached to 

 „ „ . , x , pelvic girdle; j)./, pelvic tin spine; pt.f, 



is 01 iairly normal structure, the an- pectoral fin ; it, urinogenital opening. 



terior region not excessively prolonged 



(Fig. 406). The body is elongated, scaleless, and usually provided with 



The Sticklebacks are 

 modified of this sub-order. 



