from tlic South Paci/ic. 269 



monstrated that this form, while presenting all the characters 

 essential to SagiUa, possessed others which seemed to entitle 

 it to be ranked as a species distinct from all those that had 

 been previously described. 



The most recent and exhaustive synopsis of this genus is 

 given in the pages of the ' Quarterly Journal of Microscopical 

 Science ' for 1856, by Prof. Busk. In this synopsis Mr. Busk 

 gives the characters of seven distinct species ; with none 

 of these, however, have I found it possible to associate the 

 form to be here introduced. 



This species, for which I shall here propose the name of 

 Sagitta tricuspidata (for reasons to be hereafter explained), 

 is of large dimensions, measuring very little short of an inch 

 and a half in its entire length ; in regard to size it approaches 

 S. lyra^ but it is found, on closer comparison, to be very 

 distinct from that species. In Sagitta lyra the two pairs of 

 lateral fins are described as being apparently continuous 

 with each other, while at the same time the portions belong- 

 ing to the anterior set are much larger than those belonging 

 to the posterior ones, and extend far forward. In 8. tri- 

 cusjridata^ on the other hand, the two pairs of fins are dis- 

 tinctly separate, and the anterior ones do not extend beyond 

 the posterior half of the lateral margin of the animal's body, 

 and are of smaller dimensions than the two hinder ones. In 

 this respect it seems more closely to resemble Sagitta hipunc- 

 tata ; but in the armature of the cephalic region, which forms 

 the most striking and important character of this species, it is 

 found to differ essentially, not only from the two species al- 

 ready referred to, but from all Saxjittoi that have been hitherto 

 described. 



In all these this armature is described as consisting of two 

 elements : — in the first place, of an outer series of large curved 

 corneous hooks or " falces," which are transversely movable, 

 and bound the lateral margin of the head on either side ; and, 

 in the second place, of an interior set of smaller hooks or 

 " denticles," disposed in two series, one behind the other, on 

 either side of the median line, and immediately in front of the 

 buccal orifice. 



In >S'. tricuspidata the large lateral falces are greatly deve- 

 loped, as indicated in the accompanying woodcut; but the 

 interior series or denticles are almost entirely aborted, or, at 

 most, represented in a very rudimentary condition — the only 

 structures in any way homologous to these being, first, 

 three stylate setaj set on a slightly raised prominence situ- 

 ated on either side of the anterior portion of the head (see 

 fig. 2, a), and, again, a single solitary seta occupying a position 



