220 HENRY C. TRACY 



MATERIAL AND METHODS 



Adult specimens of the following common American species of 

 Clupeoids were examined: Alosa sapidissima (shad), Pomolobus 

 pseudoharengus (alewife), Pomolobus aestivalis (summer her- 

 ring), Pomolobus mediocris (hickory shad, fall herring), Bre- 

 voortia tyrannus (menhaden). The shad specimens were 

 bought in local markets; specimens of the other species were 

 obtained from the Marine Biological Laboratory where they had 

 been preserved in 10 per cent formalin. 



In addition to the ordinary dissection methods, decalcified 

 heads of Pomolobus pseudoharengus were embedded in celloidin 

 and cut in various planes in sections 50 n in thickness. This 

 method was particularly useful in the study of the condensed, 

 perilabyrinthine tissue and the canals contained within it. For 

 the study of these structures by dissection, A. sapidissima is the 

 most favorable of these species, because the perilabyrinthine 

 structures are larger and better defined than in the smaller species. 

 For the demonstration of the perilabyrinthine spaces, some use 

 was made of the cedar oil and chloroform method (Locy, '16, 

 p. 542). 



In addition to adult specimens of the above-named species, 

 there were also available a few young specimens of Brevoortia 

 tyrannus and about thirty stages of Stolephorus mitchilli (an- 

 chovy) ranging from 3 mm. up to half-grown specimens. These 

 were sectioned in various planes and stained by standard methods. 

 Most of these specimens I obtained some years ago from the 

 lobster rearing cars at the Experiment Station of the Rhode 

 Island Commission of Inland Fisheries, then under the director- 

 ship of Dr. A. D. Mead, of Brown University. 



TERMINOLOGY 



The remarkable series of spaces and differentiated tissues 

 which are described in detail in the body of this paper are un- 

 doubtedly modifications of the perimeningeal tissue which per- 

 vades the intracranial region outside the meninx primitiva. In 

 discussing them, it is not easy to select terms which have an 



