FOSSIL AND RECENT 203 



is always beneath the orbit or the lateral ethmoid. The trunk of albuloids is rounded 

 and the head is nearly as broad at the occiput as it is deep. 



Trends and characters seen in the more 'advanced ' albuloids include the following : 

 i. Enlargement and modification of the sensory canals, 

 ii. Roof of the neurocranium with anterior ridges and troughs, particularly in 



the snout region, which is slightly elongated, 

 iii. Hyopalatine series long, shallow, with an anteriorly directed quadrate. 

 The symplectic is large and the palatine has processes articulating with both 

 the mesethmoid and the lateral ethmoid, 

 iv. Vomer reduced. 



v. Mouth small, inferior and specialized for bottom feeding. 

 vi. Premaxilla forming much of the oral margin of the upper jaw, maxilla 

 reduced, with its head inturned and moving independently of the ' fixed ' 

 premaxilla. A single supramaxilla. 

 vii. Differentiation and modification of the palatal and basibranchial dentition, 

 viii. Specialization of the dentition associated with the gill arches. 

 ix. Dentary with a strongly developed, inflected ventral border, and correlated 

 with this a reduction in size of the gular plate which may (Albula) be 

 vertical. 

 x. Dermal jaw dentition of fine needle-like teeth. 



xi. Modification of the caudal skeleton, involving reduction in the number of 

 epurals, hypurals and uroneurals. 

 The albuloid neurocranium is distinctive in showing a slightly protruding snout 

 which, in dorsal view, is narrow and marked by ridges and troughs associated with 

 the enlarged supraorbital sensory canal. The Osmeroididae generally have no promi- 

 nent ridges on the roof but Osmeroides latifrons shows a neurocranial roof much like 

 later albuloids. Enlargement of the sensory canals is seen throughout the group. 

 In the basal Osmeroides lewesiensis the cephalic sensory canals run entirely within 

 the bones, opening to the surface by pores. In 0. latifrons the supraorbital canal 

 opens to the surface above the lateral ethmoid and continues forward in a shallow 

 trough, which probably also contained the nasals. In this respect this form resembles 

 the Albulidae. In the Pterothrissidae the supraorbital canal runs within bone for 

 a short distance only ; thus in Pterothrissus the canal opens above the posterior 

 half of the orbit. The otic canal of the Pterothrissidae and of Lebonichthys among 

 the Albulidae is only partially covered by bone. 



In the Albulidae, and to a greater extent in the Pterothrissidae, the infraorbital, 

 mandibular and preopercular canals are only partially covered by bone. The open 

 infraorbital canal results from the coalescence of adjacent sensory pores, a trend 

 which is first seen in 0. latifrons. The enlargement and opening out of the man- 

 dibular canal has also taken place within Osmeroides. Thus 0. lewesiensis and 0. 

 levis show an intramural canal, while the only portion of the mandibular canal 

 contained within bone in 0. latifrons is in the articular. In both the Albulidae and 

 the Pterothrissidae the mandibular canal lies in a shallow groove, never passing 

 through bone. Associated with enlargement of the mandibular canal, the ventral 

 edge of the dentary becomes inturned so that in the Albulidae and Pterothrissidae 



