THE SKULL OF CH13LEKA, 



107 



prolonged between tbe nasal sacs as a, narrow rostral stalk in 

 Acanthias, but as a wide internasal plate in Pristiurus, tbe two 

 structures evidently being, as tbey are said by Sewertzoft* to be, 

 strictly homologous. Anterior to the nasal sacs this stalk or 

 plate expands laterally, on either side, and in Acanthias forms 

 the rostral plate of Sewertzofi's descriptions, a median keel- 

 shaped ridge projecting ventrally from its ventral surface and 

 being continuous with a similar ridge on the ventral surface of 

 the rostral stalk. In Pristiurus the rostral plate is said by 

 Sewertzoff to be represented in the three bars of the rostral 

 basket, the keel of the plate of Acanthias representing the middle 

 bar of the basket of Pristiurus, and the lateral portions of the 

 plate representing the lateral bars. This I consider to be an 

 erroneous interpretation of the conditions, the rostral plate of 

 Acanthias certainly including, with the three bars of the rostral 

 basket of Pristiurus, the narrow strip of cartilage that, in the 

 latter fish, runs laterally immediately anterior to the nasal sac 

 and is called by Sewertzoff the cartilage c. This cartilage c is 

 said by Sewertzoff to not yet be developed in the oldest embryos 

 of Acanthias examined by him, but to be found in the adult as a 

 bar of cartilage that cuts the fenestra olfactoria of embryos into 

 two parts, the lateral one being the definitive foramen olfac- 

 torium, and the mesial one the basal communication canal of 

 Gegenbaur's (1872) descriptions of the adults of certain of the 

 Selachii. The large perforation of either side of the floor of the 

 rostral basket of Pristiurus is accordingly said, in accord with 

 • Gegenbaur's earlier conclusion, to be a basal communicating canal 

 strictly similar to the one found in the adult Acanthias, but 

 greatly enlarged and shifted forward ; this large perforation 

 of the rostral basket of this fish, and the large opening that 

 actually transmits the nervus olfactorius, thus together repre- 

 senting the fenestra olfactoria of embryos of Acanthias. In an 

 earlier work (A His, 1913), I came to the conclusion that these 

 perforations of the rostral basket of the Carchariidas and 

 Scylliida? were not basal communicating canals, and that they were 

 " probably . . . simply defects in the cartilage (of the rostrum) 

 due to the presence of the large number of ampulla? found in 

 this region in these fishes," and this conclusion I consider to be 

 confirmed by conditions that I have since found in one specimen 

 of Acanthias blainvillii. In this specimen there is a basal com- 

 municating canal in normal position on each side of the head, 

 arid on one side of the head the rostral process is perforated by 

 an irregular opening certainly strictly homologous to the per- 

 forations found in the Carchariida? and Scylliidse ; and there are 

 numerous ampullae lying opposite the perforation. 



The cartilage c of Pristiurus thus certainly corresponds to a 

 part of the rostral plate of Acanthias, and the two cartilages of 

 Pristiurus, one on either side of the head, can, for convenience, 

 be together called the rostral plate, although they represent only a 

 part of that plate as found in Acanthias. This plate and its stalk, 



8* 



