490 ALBERT W. BROWN. 



attention to some obscure references of great importance 

 shortly after I had begun to work on this form. 



In O. F. Miiller's * Vergleich. Anat. der Myxinoiden ' a 

 statement occurs to the effect that the fourth ventricle of the 

 brain contains many Diplosomids, and this is the earliest 

 statement yet found. My discovery therefore is not new. 



George Gulliver, in papers dealing with the blood-corpuscles 

 of the lamprey published nearly thirty years ago, noticed 

 these forms, and stated their occurrence. He promised a 

 further description, but, so far as I can discover, did nothing 

 more than give them the rather fancy name of Neuronaia 

 lampetrse. The genus Neuronaia (or Neuronaina^) was 

 founded by Goodsir for a parasitic form from the nerves of a 

 shark which he called Neuronaia munroi, and Gulliver's 

 association of his species with this appears to have no justifica- 

 tion beyond a distant similarity of habitat. 



The name Neuronaia is in every way an undesirable one. 

 The form here described is immature, and has therefore no 

 right to be considered even a new species till its adult form 

 has been determined. This I have, unfortunately, not been 

 able to do, but it probably belongs to an already named 

 Holostomid genus. In the second place, it is now the custom 

 to call such immature forms by the generic name of Tetra- 

 cotyle, and I propose therefore, in order to bring this form 

 into harmony with present-day nomenclature, to call it Tetra- 

 cotyle petromy zontis. The most confirmed devotee of 

 the laws of priority can scarcely object to this, seeing that 

 Gulliver gave the form a name without attempting to deter- 

 mine its real position. If every fancy name is to be retained, 

 all hope of reducing our zoological nomenclature to a simple 

 uniform system must be abandoned. 



I may here point out the remarkable similarity in appear- 

 ance between Tetracotyle petromyzontis and the form 

 known as Diplostomum volvens.^ Indeed, at first I was 



' There appears to be some irregularity iu the spelling of this word. 

 * Nordmanu's figure of Diplostomum volvens is reproduced in the 

 •Cambridge Natural History,' vol. ii, p. 64, fig. 31. 



