76 E. LE ROY MICHAEL. 



some of these characters but, as yet, they are in a comparative 

 form lacking in that element of exactness and definition so neces- 

 sary for accurate classification by one unfamiliar with the group. 



Another important character is present in the vestibular ridge. 

 The presence of high or low, numerous or few, blunt or acute 

 papillai are matters that need more attention in the future. The 

 position and height of the papilla should be compared to the 

 position and height of the tooth, and similarly their number 

 should be compared to the number of teeth. Comparison of the 

 extent of the ridge with that of the tooth row proves of some 

 importance ; whether it is longer or shorter than the tooth row, 

 and if shorter how many teeth project beyond ; whether it ter- 

 minates abruptly or gradually, in a lateral process or not. 



The fins offer good characters if the specimen is well preserved. 

 The presence or absence of a second pair of lateral fins serves to 

 distinguish between Sagitta and the other two genera. Relation 

 of the anterior to the posterior fins in length and breadth has 

 been found to be a useful character. In Sagitta licxaptcra, S. 

 inacroccphala, S. mghxta, S. bipniictata, S. robusta, S. scrratodcn- 

 tata, and vS. planctonis the posterior fin is longer then the anterior 

 fin ; in Sagitta fcrox, S. pnlclira, S. cctesios, S. zvhartoiii, and 5". 

 siboga the posterior fin is shorter ; in Sagitta bcdoti, S. eiiflata, S. 

 regularis, S. fiircata, and 6". decipiens both fins are the same 

 length. Here again, it is necessary to know how much variation 

 to allow for those in the third group, and to accurately determine 

 this requires measurements of a large series of individuals of each 

 species. 



The distance from the anterior to the posterior fin, as measured 

 in per cent, of the total length of the individual, may be made of 

 considerable service. From the table we see that, in Sagitta 

 bcdoti, this distance measures but 5.4 per cent., the least I have 

 found, except in the case of Sagitta zvhartoni where the fins are 

 confluent, while 6". siboga measures approximately 15 per cent. 

 The other species are distributed between these extremes. While 

 the measurements are not extensive enough to be of much value, 

 still we can separate the species into two groups ; those in which 

 this interval is more than eight per cent, and those in which it is 

 less than eight per cent. In making this separation we still 



