— 200 — 



the Cyprinidae. According to that author ('91, p. 574), a well developed myodome is found in most 

 of these fishes. In Cobitis, Misgurnus, Nemachilus and Acanthophthalmus, all of which have sraall 

 eyes, the myodome is said to have undergone retrogression. The weak muscles of the eye are said to 

 all arise in the postero-ventral corner of the orbit, none of them entering the cranial cavity. The 

 anterior edge of the proötic is said to be formed of two lamellae one lying above the other, the dorsal 

 one ending with a free anterior edge, while the ventral one forms the floor of the cranial cavity. 

 The slit-like space between the two lamellae, although it lies considerably posterior to the hind end 

 of the orbit, is said to unquestionably represent a myodome that has undergone reduction. Reference 

 to the figures of Cobitis fossilis (pl. 29, fig. 12) shows that the dorsal lamella must be a rudimentary 

 mesial process of the proötic, and as such Sagemehl doubtless considered it, although he does not 

 definitely describe it as such. Anterior to this process, there is said (1. c, p. 549) to be a wide rhom- 

 boidal hypophysial fenestra. The nervus abducens, in those Cyprinidae that are said to have no 

 myodome, and of which Cobitis is one, is said not to perforate the mesial process of the proötic, but 

 to apparently issue by the large optic fenestra. The condition of the myodome is thus here closely 

 similar to that in Lepidosteus; that posterior portion of the canal that I have referred to as the 

 saccular space, existing in a reduced condition, well within the brain case, widely separated from 

 the orbit, and in no direct relation whatever to any of the eye-muscles. This condition is thus the 

 reverse of that found in Dactylopterus, Gobius and Gadus, in which fishes this part of the myodome 

 has been crowded out of the cranial cavity into the orbit. 



Ameiurus can now be considered, and of this fish I have one skull, prepared some years ago 

 and partly disarticulated, and a few small alcoholic specimens. Of Ameiurus, Mc Murrich says: 

 „Below the proötics, where they meet in the middle line below, and between them and the anterior 

 portion of the basioccipital above, and the parasphenoid below, is a small cavity. This is the almost 

 aborted rudiment of the canal for the orbital muscles, which is largely developed in many fishes, 

 but absent or rudimentary in Silurus, Ameiurus, Gadus, Lophius etc." 



This description of the conditions in Ameiurus is not very satisfactory, and it is not explained, 

 either here or elsewhere in the descriptions, that this so-called rudimentary myodome is widely 

 separated from the orbit and out of all relation to the eye-muscles. Yet such is the case. In the 

 anterior three-fifths, approximately, of its length, the ventral edge of the proötic does not meet its 

 fellow of the opposite side, a wide hypophysial fenestra, closed ventrally by the parasphenoid, being 

 left between the two bones. Posterior to this fenestra, the ventral edges of the proötics meet in the 

 middle line, and the two bones there form, on the floor of the cranial cavity, a prominent transverse 

 bolster which has closely the position of the cross-canal of Lepidosteus; and it is certainly in this 

 bolster that McMurrich found the small cavity that he considered as a rudimentary myodome. In 

 two of my small specimens there was no indication whatever of such a cavity ; the bolster there being 

 completely filled with cartilage, but having, on its anterior surface, a slight median depression which 

 doubtless lodged the Saccus vasculosus. In one other specimen, there was a median cavity extending 

 under, rather than into the bolster, while in a fourth specimen there was a smaller but similar cavity 

 on one side only of the median line. The pituitary body lies immediately anterior to the bolster, 

 directly above the hypophysial fenestra, and a vein that apparently drains the pituitary region 

 begins here and runs directly forward along the floor of the cranial cavity, soon separating into two 

 parts, one on either side. The venous cross-comissure of Lepidosteus thus here seems replaced by 

 a median vein, this being a Variation in detail but not in principle. The eye-muscles all have their 



