THE SKULL OF CHnLEP.A, 107 



prolongerl between the uasal sacs n.s a, iiiiriow rostral .stalk in 

 Acanthias, but as a wide interiiasal plate in Pristiarus, the two 

 structures evidently being, as they are said by SewevtzofF to be, 

 strictly homologous. Anterior to the nasal sacs this stalk or 

 plate Expands laterally, on either side, and in Acanthias forins 

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

 shaped ridge projecting ventrally from its ventral siiiface and 

 being continuous with a similar ridge on the ventral surface of 

 the rosti-al stalk. In Fristmrus the i-ostral plate is said by 

 Sewertzoft' 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 Fristiurtis, 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 Fristiurus, the narrow strip of cartilage that, in the' 

 latter iish, runs laterally immediately anterior to tlie nasal sac 

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

 said by SewertzofF to not yet be developed in the oldest embryo.s 

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

 y)ar of cartilage that cvits the fenestra olfactoiia of embryos into 

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

 torium, and the mesial one the basal communication canal of 

 Gregenbaur'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 Fristiurus 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 peiforation 

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

 actually transmits the nervus olfactorius, thus together repre- 

 senthig the fenestra olfactoria of einbrj^os of Acanthias. In an 

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

 perforations of the rostral basket of the Carchariidte and 

 Scylliidas Avere not basal communicating canals, and that they weie 

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

 due to the presence of the large number of ampullfe found in 

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

 confirmed b}^ conditions that I have since found in one specimen 

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

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

 and on one side of the head the rostral pi^ocess is perforated by 

 an irregular opening certainly strictly homologous to the per- 

 forations found in the Oarchariidfe and .Scjdliidpe ; and there are 

 numerous ampullae Ij'ing opposite the perforation. 



The cartilage c of Fristiurtts thus cei'tainly corresponds to a 

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

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

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

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



8* 



