REVIEW OF HIPPOCAMPUS — GINSBURG 505 



flexing upward the end of the tail with the tip of a dissecting needle. 

 It is possible that if the fish were macerated and the last one or two 

 segments determined with absolute accuracy, the numbers would 

 difTer slightly from those given in this report. However, this is 

 manifestly impracticable, and the difference, if any, would be very 

 slight. In employing the method of counting here described, one 

 should also note that there is sometimes a transverse groove on the 

 center of many tail segments on the ventral side. Although sometimes 

 confusing, this groove is nearly always much shallower than the one 

 marking the boundary between the segments, and with a little practice 

 the distinction between the two sorts of grooves is readily made. The 

 grooves often disappear in the anterior part of the tail, but the trans- 

 verse ridges and spines, at the center of the segments, become promi- 

 nent anteriorly and aid in the accurate determination of the number 

 of anterior segments. \\'liere the grooves on the ventral side are indis- 

 tinct, the tail may be uncoiled and its lower side placed against a glass 

 slide. The pressure of the slide against the natural tendency of the 

 tail to recon brings out the boundaries between the segments on the 

 dorsal side with more or less prominence, and they may thus be 

 counted on that side. 



Modification in structure of segments in region between trunk and 

 tail. — Special attention should be called here to some structural char- 

 acters of the species of Hippocampus that liitherto have received only 

 scant attention but that are of considerable importance in the proper 

 differentiation of the species in showing theh probable relationship 

 and in distmguishing the subgenera. I have reference to the modifi- 

 cation in the structure of the first caudal and last trunk segment; 

 sometmies the last two trunk segments are involved. The modifi- 

 cation of the first caudal segment was briefly referred to in the pre- 

 ceding paragraph in connection with the accurate determination of 

 the number of caudal segments. The modified nature of all the 

 segments in that region will now be discussed in greater detail. 



We are concerned herein chiefly with the external structure of the 

 fish as seen without any dissection, for the practical purpose of dis- 

 tinguishing the species. For a detailed description of the minute 

 structure of the segments, the reader is referred to Rauiher " or to 

 Duncker.^^ However, the gross external structure may be better 

 understood when the detailed anatomy is considered. Briefl}', a 

 typical caudal segment consists of four plates, two dorsolateral and 

 two ventrolateral. Each plate consists of two wings bent at an angle 

 with a lengthwise ridge along the bend. Another, a transverse and 

 blunter ridge, occurs along the middle of the plate. Wlien joined the 

 four plates somewhat overlap, are loosely ankylosed by projecting 



" Die Syngnathiden des Oolfes von Neapel, pp. 68-74, pi. 7, 1925. 

 "Syngnathideu-Studien. Mitth. Naturh. Mus. Hamburg, vol. 25, pp. 18-20, 1908. 



