THE DIFFERENTIAL CHARACTERS OF THE SALMONID.E 

 AND THYMALLID.^^:. 



By TiiEoDoKK Gill, M. D., Ph. D. 



In 1S85 the name ThymalUdcv was published, but witliont defluition. 

 I luxve on several occasions been reciuested to give the distiiK tive char- 

 acters of the family, and have done so orally. A detailed exposition 

 has been postponed in the hope that I might be able to study the anat- 

 omy of tbe related forms. As no immediate prospect of doing so is 

 offered, however, I submit diagnoses of the iS((lnioni<1a'a>s now restricted 

 and the ThyviaUida: 



In 1871 Prof. Cope, in his system of teleostomous fishes, named as 

 fiimilies of his order of Isospo)i(lyli, among others, the families Salmon idw 

 and Coregonidce. The Isospondyli with a diphycercal tail and "basis 

 cranii double" are divided among thos«^ with "(^0 parietals united," 

 and '•'{<((() parietals separated by supraocci})ital." The former [a) 

 include the Hyodoiitida', Alhulidcv, Ulo^ndic, Aulopid<(\ Corecionid.^, 

 Lutodirida', Sanridfv, and GonorliyncMdiV ; the latter {<i<i) compose the 

 Alepocephalidw, Sal^monid^e, Chirocentrida', and Clnpeidiv. 



I at lirst adopted the Salmonidw and Coregonidw in my Arrangement 

 of families, but, on examination of a skull of Coregomis shortly before 

 receiving proofs of that article, found that it did not have the " parietals 

 united," but "separated by supraoccipital, " and thus agreed Avith the 

 salmonids. I consequently replaced the name Coregonidw by Mieros- 

 tomid(e, but the printers retained the reference to Coregonidw of Cope. 



As thus intimated, the true Coregoni have the same relations of the 

 supraoccipital, parietals and frontals to each other as the typical sal- 

 monids, but there is a genus which manifests the character erroneously 

 attributed to Coregonida' by Co[)e, and that genus is Thymallus. 



ThymaUus is not only distinguished from the true Salmonine and 

 Coregouine fishes by the junction of the parietals at the middle; it has, 

 in addition, supracostal spines entirely wanting in the others; further- 

 more, the dorsal is distinguished by its greater develoi)ment, both in 

 extension and the number of rays, as well as its structure; instead of 

 only one or two simple anterior rays, as in the Salmonines and Corego- 



Procei'diugs Katioual Museum, Vol. XVI— INo. 992. 



117 



