THE MYXOSPORIDIA, OR PSOROSPERMS OF FISHES. 237 



anterior end of each series appears usually to be (is?) formed by one 

 of the pericornual nuclei. Sometimes these latter are the only ones to 

 be seen. Almost always they are the largest. Starting anteriorly with 

 these two, an increase may be traced up to G (3 on each side^), the 3 

 pairs being- often subsymmetrically arranged. In cases of deficiency it 

 is the posterior ones that are absent. These facts would seem to suggest 

 a possible origin of the series from the two large pericornual nuclei. 



Besides the structures already described, others more or less similar 

 may be seen, especially anteriorly and in the higher (presumably also 

 in the lower) focus-planes. Some of these show the same combination 

 (deeply stained granules in unstained areas) as those already men- 

 tioned, but often no surrounding unstained areas were visible. 



Vacuole: I could not detect this structure, but do not wish, on the 

 strength of the material available, to positively assert its absence. 



Habitat, etc. — Encysted immediately beneath the skin, on the external 

 (scaleless) surface of the head, never elsewhere except twice in skin of 

 body immediately behind head of Urimyzon siicetta ohlongus { = Catos- 

 tomns tuhercidatus Le Sueur, Jide Jordan and Drayton 2), chub sucker. 

 Apparently a scaly surface constitutes an almost impassable barrier 

 for this species. 



Observed on fish collected as follows: 



U. S. Nat. Mus. Cat. No. 20105. Tributaries Fox River, Mississippi. Collector, Prof. 



S. F. Baircl. Tumors very numerous on 2 specimens. Fish adults. 

 U. S. Nat. Mus. Cat. No. 20523. Kinston, North Carolina. J.W. Milner, collector. 



A single tumor on 1 fish; the latter rather young. 



This species was not found in the following: 



U. S. Nat. Mus. Cat. No. 20254. Near Piermont ( fPierpont) New York. Collector 

 Prof. S. F. Baird. Fish half-grown. 



U. S. Nat. Mus. Cat. No. 25573. Columbia, South Carolina, March 21, 1880. Col- 

 lector, Col. Marshall McDonald. 



The striking contrasts between the very great number of cysts present 

 on the fish from Mississippi and their extreme rarity upon those found 

 at the other localities is interesting. Data are, however, wanting for the 

 proper appreciation of relative potency of geographic location, tempera- 

 ture, season, and age of the fish. 



Remarks. — This species is, I believe, identical with the one described 

 by Mliller.^ Although he states the branchiai to be the principal seat 

 of this species, I have only found it imbedded under the skin covering 

 the head. The cysts found on the branchiae, besides beingdistinguished 



' I have not seen more than 3 nncleiform bodies (deep-staiued granules in the 

 midst of a non-stained area) on a side, though the number of deep-stained granules 

 may be greater, 2 being sometimes found in one unstained .space. 



- Bull. 12, U. S. Nat. Mus., pp. 100, 1 15; var. oblotujiin, fide Prof. B. W. Kvermann. 



'Miiller's description in bricrf is: 



Cysts con.spicuous, elongat(% 2 to 1 mm. long, imbedded principally under mucous 

 membrane of branchial lamolbe, also in that of the branchial chamber and in skin of 

 head of Catostomna tuberculatus from North American rivers. Cysts found in all of 

 the 3 fish examined, being in one case numerous. 



