454 DR. W. G. RIDEWOOD ON THE CRANIAL [DeC. 13,, 



Hyrtl, in his papei- on the Accessory Branchial Oi'gan of 

 Clupeoid Fishes (Denkschr. Akad. Wiss. Wien, x. 1855, ph 1. 

 figs. 2 and 3) has published figures of the branchial skeleton of 

 Ch('pea tliryssa [Meletta thryssa) and Chipea mattotoocca {Alausa 

 tyranmis), but they are of little service in the present connection. 

 Vrolik (Niederl. Arch. Zool. i. 3, 1873, pp. 268-270, and figs. 28 

 and 29) has given a short description and two figures of the 

 cranium of the Herring ; and Matthews has contributed a very 

 complete account of the whole skull of that fish, with observa- 

 tions also on the skulls of Clupea finta, Clup)ea pilchardus, and 

 Ghijyea spratUis (Fifth Rep. Fish. Bd. Scot. 1887, pp. 274-292, 

 and figs. 15, 17, and 18). Three good figures of the cranium of 

 the Herring appear in Fries' ' Scandinavian Fishes " (ed. 2, 

 by F. Smitt, vol. ii. 1895, p. 949). 



Material examined. — The description below applies mainly to 

 Clupea finta, but Glup>ea harengus^ Clupea sajndissi'ma, and Clv,pea 

 sprattus were also examined. The hyobranchial skeleton de- 

 scribed is that of Chipea harengus. All the skulls were specially 

 prepared for the purposes of the investigation. 



Cranium (text-fig. 122, p. 455). — The parietals are separated by 

 the supraoccipital. The posterior temporal groove {mastoid groove 

 of Fries, I. c.) is not roofed. Near its anterior end is an oval 

 temporal foramen (Fries, I.e. p. 947), bounded by the parietal 

 and frontal, and occupied by a fatty mass in the"| fresh fish, but 

 leading directly into the cavum cranii in the dried skull ; behind 

 it is the pre-epiotic fossa, bounded by the parietal, squamosal, and 

 epiotic, and ending blindly against the supraoccipital. The pre- 

 epiotic fossa is relatively larger in Chipea harengtos and Gltipea 

 sprattus than in Clupea finta and Chqjea sapidissima. 



In the middle of the pro-otic bone is a conspicuous bulla con- 

 taining a spherical vesicle of the swim-bladder. A second bulla 

 is present in the squamosal, but this is not visible in Clupea finta 

 3xcept by breaking open the bone ; in Clupea harengus it is just 

 visible on the surface. Chi,pea sp)rattus has no squamosal bulla 

 (Matthews, I. c, and Ridewood, Journ. Anat. and Phys. xxvi. 

 1891, p. 36, and fig. E, p. 32). In Clupea harengus the duct 

 that leads through the exoccipital bone from the pro-otic and 

 squamosal vesicles to the swim-bladder is dilated and fusiform 

 in shape. The auditory fenestra, in the ventro-lateral aspect of 

 the cranium, has an irregular outline ; it is bovmded by the 

 pro-otic, exoccipital, and basioccipital, and leads into the peri- 

 lymphatic cavity of the ear. 



The cpisthotic is moderately small ; it touches the pro-otic, 

 squamosal, and exoccipital bones, but not the epiotic. The 

 basisphenoid is small and transversely set, and its descending- 

 process is a mere spicule of bone which fails to reach the para- 

 sphenoid. The orbitosphenoid sends forward a pi-ocess which 

 meets a backwardly directed process of the united prefrontals. 

 The parasphenoid is straight, or nearly so, in Ghtpea finta, but 

 the middle part is slightly depressed in the other species examined. 



