190 SUB-CLASS (and ORDER) TELEOSTEI. 



and orbitosphenoid, and the anterior part of it generally acquires 

 a considerable vertical extension forming an interorbital septum 

 (absent in Siluroids, Cyprinoids, etc.). The ethmoid region 

 remains unossified, or at most has two bones — the lateral eth- 

 moids or prefrontals (Fig. 110 Ethl). The membrane bones of the 

 roof are parietals {Par), large frontals {Fr) and a bone over the 

 ethmoid (supraethmoidal or median ethmoid, Ethi). There 

 may be other bones, called nasals, over the ethmoidal region. 

 The parietals may touch in the middle line between the frontals 

 and supraoccipital, or be pushed apart and separated by the 

 junction of the frontals and the supraoccij^ital. In the floor 

 there is a large and important parasphenoid strengthening 

 the base of the skull and a vomer [Vo] underlying the 

 ethmoid region. The orbit is surrounded by a ring of 

 circumorbital membrane bones (not shown in Fig, 110), 

 of which the anterior is called the lacrymal. Premaxillae 

 [Jm) and maxillae [Mx) are present, and there may be a 

 jugal, but the maxillae are usually toothless and frequently take 

 no part in the formation of the edge of the mouth. The palatine 

 bar of the mandibular arch always presents osseous palatine, 

 pterygoid and quadrate elements : in front is the palatine 

 {Pal) often dentigerous ; then follows the pterygoid region 

 usually presenting three elements — the pterygoid (ectoptery- 

 goid), the mesopterygoid (entopterygoid Enp) and the meta- 

 pterygoid {Mtp) ; lastly comes the quadrate {Q) which gives 

 articulation to the lower jaw. The front (palatine) end of this 

 bar is attached to the ethmoid region, while the quadrate is not 

 attached to the cranium directly, but is supported by the 

 strong dorsal element (hyomandibular) of the hyoid arch. In 

 the lower jaws Meckel's cartilage j)ersists, being ossified proxi- 

 mally as the articular {Ar), and ensheathed by the dentary 

 bone {D) distally : in addition there is often an angular element 

 {An). In the hyoid arch there is a powerful dorsal hyomandi- 

 bular element which presents two bones, the hyomandibular 

 {Hm) and the symplectic {S). The hyomandibular bone 

 articulates with the auditory region of the cranium and passes 

 ventralwards behind the metapterygoid, while the symplectic 

 lies distally and is closely applied to the quadrate. The rest 

 of the hyoid arch consists of three ossified pieces on each side 

 ,(Fig. Ill) — the epihyal (c), ceratohyal (6) and hypohyal. The 



