FOSSIL AND RECENT 121 



the ventral end of the bone, where it curves sharply forwards resulting in an oblique 

 ventral border. The suboperculum and interoperculum have the same form as in 

 0. lewesiensis. 



Squamation. Only a few of the anterior scales are known. The scales are thinner 

 than in the type-species although of the same relative size. Each scale is nearly 

 circular. The anterior field is marked by random fine granulations. In the dorsal 

 and ventral fields the granulations become defined as circuli while in the posterior 

 field they form a network of rugae and thus differ from those in 0. lewesiensis 

 which are set in radiating fashion. The scales of 0. latifrons are without anterior 

 radii and the anterior margin is complete ; a few of the anterior body scales of the 

 type-species are also without radii. The lateral line scales may be identified by a 

 small notch in the posterior margin, as in 0. lewesiensis. 



Affinities of Osmeroides latifrons. Osmeroides latifrons is an advanced species 

 of the genus. Many features of 0. latifrons are interpreted as the initiation of trends 

 which are characteristic of albulids and pterothrissids. For instance, the cephalic 

 sensory canals of 0. latifrons are considerably more open than those of 0. lewesiensis 

 or 0. levis, but are never cavernous as in the Albulidae and Pterothrissidae. Inter- 

 mediate conditions of other characters are seen in the snout, hyopalatine series and 

 the dentition, and are discussed in relation to the evolution of the Albuloidei (pp. 

 202-210). Despite certain resemblances to the Albulidae and Pterothrissidae, 0. 

 latifrons retains primitive characters of the Osmeroididae such as the terminal 

 mouth, the incorporation of the first centrum into the neurocranium, the unexpanded 

 ectopterygoid process, the presence of many ectopterygoid teeth and the unspecialized 

 dentition, particularly that associated with the gill arches. In all, the known 

 osteological features of 0. latifrons show a closer resemblance to other species of 

 Osmeroides than to any other albuloid genus. Those points in which 0. latifrons 

 differs from 0. lewesiensis and 0. levis concern degree of development rather than ab- 

 solute difference and it is therefore difficult to justify exclusion of 0. latifrons from 

 the genus. 0. latifrons is known only by incomplete cranial remains and until more 

 complete specimens are available it is retained in Osmeroides. Osmeroides latifrons 

 is most nearly related to 0. levis. With this species it shares such features as smooth 

 cranial bones, a very shallow otic region of the neurocranium with large otic bullae, 

 a membranous interorbital septum and small dentary teeth which are decidedly 

 pointed. With 0. lewesiensis it shares but one character, a very broad parasphenoid. 



The cranial anatomy of 0. latifrons suggests a morphotype for the Albulidae and 

 Pterothrissidae. Evolution of the latter fishes from 0. latifrons would primarily 

 involve loss of the medial ridge of the sub-epiotic fossa ; deepening of the trough 

 upon the skull roof ; a general enlargement of the sensory canals with a reduction 

 in their bony protection ; reduction or, in the case of the Albulidae, loss of ectoptery- 

 goid teeth ; expansion of the ectopterygoid process to contact the infraorbitals ; 

 development of an inferior mouth ; development of an inflected dentary margin 

 with consequent approximation of the jaw rami in the ventral mid-line and reduction 

 or loss of the gular plate. In three features 0. latifrons resembles the Albulidae 

 rather than the Pterothrissidae ; the position of the quadrate/mandibular articula- 

 tion, the broad parasphenoid and the irregular shape of the parietals. The first 



