_- 12 — 



to think is not strictly correct, for the fossa in some of these latter fishes is certainly similar to that 

 in Scomber, which, as above explained, is something more than the equivalent of that in Sclerognathus 

 and Diplophysa. Sagemehl further says that the fossa in the fishes above mentioned has arisen wholly 

 independently of that in Sclerognathus. This also seems to me incorrect. Sagemehl himself says that 

 the fossa in Amia represents a primitive condition. From that fossa, the fossa of Scorpaena is readily 

 and directly derived; and from the fossa of Scorpaena that of Sclerognathus would be directly derived 

 by the simple reduction, which has actually taken place in the latter fish, of the extrascapular and 

 suprascapular bones. And from the condition found in either Scorpaena or Sclerognathus that in 

 Scomber would be produced by the continued anterior Prolongation of the muscles that fill the groove, 

 the muscles passing dorsal to the parietal and frontal bones instead of ventral to them. If the muscles 

 passed beneath the parietal and frontal it would give rise to the conditions found in Macrodon and 

 Elops. From the condition found in Sclerognathus, also, that in Nemachilus, Misgurnus, Cobitis and 

 Acanthophthalmus would be derived by a simple constriction of the groove, due to the encroaching 

 incrowth of the pterotic and epiotic. It seems almost unnecessary to State that the conditions found 

 in these latter fishes is certainly not a primary one, the trunk muscles here being seen in process of 

 excavating the fossa in the solid bones of the skull. 



In Gasterosteus, according to Swinnerton ('02), the temporal groove is shallow and 

 wholly uncovered, the extrascapular (supratemporal) being said to lie superficial to the trunk 

 muscles but wholly posterior to the hind edge of the skull. Swinnerton considers the groove in 

 this fish as similar to that in Amia. It is, on the contrary, similar to, but less developed than 

 the groove in Scomber, in which fish the groove also lies wholly external to, instead of internal 

 to, the parietal. A similar groove is also found in all the Clupeoid fishes described by Ridewood 

 ('04c), excepting only Chanos, but there is, in these fishes, a related temporal foramen not found 

 in Scomber or Gasterosteus. 



In Gymnarchus the temporal fossa undergoes special development and acquires relations to 

 the auditory organ, as I have recently shown ('04); and, judging from Ridewood's descriptions, the 

 same may be true of the other Mormyridae and of the Notopteridae. In these latter fishes, Ridewood 

 ('04b) does not describe either a temporal fossa or groove, but his figures show such a groove, appar- 

 ently similar to that of Scomber, this groove being more or less roofed by an extrascapular (supra- 

 temporal) bone or bones, and having, in its mesial wall, a large opening. This opening Ridewood 

 describes as a lateral cranial foramen, and he considers it ('04a, p. 61) as the possible homologue of 

 the preepiotic fossa of his descriptions of Clupea. 



In Gonorhynchus, according to Ridewood ('05b), there is no Suggestion, even, of a temporal 

 groove; but^ judging from the figures given, it would seem as if there might there bo a very shallov 

 groove of the kind found in Scomber. 



It may here be stated that, in Notopterus, Ridewood says that the extrascapular (supratemp- 

 oral) .,does not carry the sensory tube", the supratemporal cross-commissure lying anterior to that 

 bone. Bridge ('00, p. 517) however says that this sensory canal of Notopterus is „strengthened", 

 along its lateral and inner walls, by ,,two longitudinally arranged, thin, semi-cylindrical bones, or 

 sensory ossicles"; which ossicles must accordingly be, or belong to, the extrascapulars. Be this as it 

 may, the Variation seems morphologically unimportant, for the extrascapular does not actually 

 „carry" the canal in Gymnarchus either, the bone lying wholly superficial to the canal. 



