No. 2.] AMIA AND TELEOSTS. 93 



laterale, the other two the " angewachsenen " occipital arch of 

 the fish. The posterior foramen is said to resemble exactly 

 the foramina found in the dorsal arches of the free vertebrae, 

 and Sewertzoff hence concludes that it unquestionably gives 

 passage to a spinal-like nerve. This nerve is said by him to 

 "belong" to the so-called occipital arch of the fish and to indi- 

 cate, with the next anterior nerve, that that arch is formed by 

 the fusion of two dorsal vertebral arches instead of representing 

 but one such arch, as Gegenbaur asserts. The drawing which 

 accompanies Sewertzoff's descriptions seems to me to show, 

 beyond question, that the nerve here under consideration, and 

 the following spinal ones, each innervate the muscle-segment 

 that lies immediately posterior to the arch the nerve in question 

 perforates. The last nerve that perforates the skull is, accord- 

 ingly, a post-occipital and not a spino-occipital one, exactly 

 as in Scomber ; and as it seems, both from Sewertzoff's figure 

 and descriptions, to have been but recently, and still incom- 

 pletely, incorporated in the skull, this may account for its appar- 

 ent absence in the specimen described by Gegenbaur. This 

 nerve in Lepidosteus is considered by Sewertzoff as the homo- 

 logue of the last spino-occipital nerve in Amia, and the two ver- 

 tebral arches said to be represented in the single occipital arch 

 of Lepidosteus are accordingly considered as the homologues 

 of the two partly assimilated occipital arches of Amia. If the 

 posterior spino-occipital nerve of Lepidosteus is, as it seems 

 to be, a post-occipital nerve, this comparison is evidently not 

 correct. 



With Acipenser, so fully described by Sewertzoff, I am un- 

 able to make any comparison, the embryos of Amia that I have 

 as yet investigated not having been sufficiently young to show 

 whether or not a certain number of the anterior postotic somites 

 disappear in this fish without giving origin to permanent muscle- 

 segments. It seems to me, moreover, that there is some con- 

 fusion in Sewertzoff's descriptions. On pages 224-8 of his 

 memoir he says, that in stage B of Acipenser the first myo- 

 tome posterior to the ear capsule still exists, but is relatively 

 much reduced in size. The first dorsal root in the specimen 

 representing this stage lay opposite the fifth myotome on the 



