502 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 83 



termined above, to the spur under the base of the dorsal fin, and count 

 the spurs or spines on the upper ridge, forward. Wliere the spurs or 

 spines fade out anteriorly, the transverse ridges on the back of the 

 segments in front of the dorsal fin always saliently mark their 

 segments, 



Wliat appears on external examination to be the first trunk seg- 

 m.ent, the one bearing the pectoral, is a compound segment and is 

 composed of three me ta meres, corresponding to the first three verte- 

 brae, according to Rauther.^" The posterior one of these three meta- 

 meres is readily identified b}'" having a spur on the upper longitudinal 

 ridge and a transverse ridge on the back, as in the following segments. 

 It was consequently included in the counts recorded in this study. 

 The more or less reduced elements of the anterior two metameres are 

 intimately fused with the third, and the three appear externally as a 

 single somewhat irregular segment, bearing the pectoral fin. This 

 compound segment was uniformly counted as one throughout this 

 stud}'', by including the spur or ridge of the third metamere with the 

 following ones, as stated. The anterior two metameres were not in- 

 cluded separately in the count, although their presence usually may be 

 detected externally by the two nuchal, more or less spurred, plates. 

 (The coronet may possibly represent a remnant of still another 

 primitive metamere.) 



Figure 54.— Exoskeleton of Hippocampus hippocampus (after Rauther) 



cor.: Coronet. 



nu. pi.: Nuchal plate. The two nuchal plates on the mid-dorsal surface form parts of the reduced anterior 



two segments, corresponding to the first two vertebrae, according to Rauther. These two segments 



are intimately fused with the third. All three appear externally as one irregular segment, bearing 



the pectoral fin, and were counted as one in this study. 

 tr. seg. 1: The first trunk segment as arbitrarily and uniformly adopted for the purpose of this study. 

 antp. tr. seg.: Antepenultimate trunk segment, showing the typical structure of a trunk segment as follows: 



up. pi., upper plate; med. pL, median plate; I. I. pi., lower lateral plate; m. v. pL, midventral plate. 



The first three plates are paired and the last is unpaired, a typical trunk segment thus being septangular 



in cross section. 

 p. tr. seg.: Penultimate trunk segment. Note that this segment is septangular like the preceding, except 



that the upper plate is pushed upward to occupy a position nearly in a line with the extra plates on the 



following two segments. The penultimate trunk segment sometimes also has an extra plate and is 



novemangular, depending on the species or on individual variation. 

 I. tr. seg.: Last trunk segment. This segment lacks the midventral plate and has an extra plate, ex. pL, 



superimposed over that plate, which is the homologue of the upper plates of the preceding segments. 



This segment is always octangular, except in zosterae, where it is often hexangular, and sometimes 



asymmetrical in other species as an infrequent individual variation. 

 caud. seg. 1: First caudal segment. This is like the following segments except that it has an extra plate 



on top, and is thus hexangular. The ab.sence of an extra plate on this segment is usually a specific 



character, sometimes an individual variation. 

 cavd. seg. S: Second caudal segment, showing up. pi., upper plate, and t. pi., lower plate; both are paired, 



and a typical caudal segment is quadrangular. 

 / . sp.: Last spur on lower lateral ridge, unmistakably marking the last trunk segment externally and nearly 



always present, except in occasional specimens having the last trunk segment asymmetrical; sometimes 



obsolescent. 

 w,: Wing from lower plate of last trunk segment extending inwardly to meet its fellow from the opposite 



side behind the anal fin. 



•• Die Syngnathiden des Qolfes von Neapel. Fauna and Flora des Qolfes von Neapel, Monog. 36, 1925. 



