OSSEOUS SYSTEM. 195 



many Fishes of the Eel family ; and it is here that their structure 

 exhibits an interesting series of gradations. Their form appears to 

 be of the simplest kind in Muracnophis, where, upon either side, 

 there lies beneath the skin behind the gills a single bone, sloped 

 out anteriorly and surrounded by muscles, which supports no fin. 

 A similar pair of very delicate styliform bones, abutting together an- 

 teriorly, are situated behind the branchial apparatus in Sphage- 

 branchus ; these bones are more largely developed in Synbranchus 

 (Unibranchapertura, Lacep.), where they form a girdle that corre- 

 sponds somewhat to the clavicles. In Mursena each of these bones 

 is already divided into two, the uppermost of which may be viewed 

 as a rudimentary scapula, while the remaining divisions enter into 

 the composition of the fin an arrangement approximating the usual 

 structure which is as follows : There is generally situated most 

 superiorly one bone, or occasionally several, that represent the 

 scapula. The uppermost is usually bifurcated, and unites by means 

 of its two processes with the occipital bone ; it is rarely, however, 

 firmly united to the skull by suture, but usually by ligament alone. 

 To this, or to a second small bone, succeeds interiorly one that is 

 very much larger, presents a slight semiluriar excavation in front, 

 and comes in contact with that of the opposite side, so as to form a 

 girdle for the anterior extremities, which gives off the main support 

 of the fin. The bone, which corresponds to the single one in Mu- 

 raenophis and Synbranchus, has been named, perhaps justly, the 

 anterior clavicular. 'It is more difficult to admit the title of posterior 

 or coraco-clavicular applied to the next bone, as the analogue of the 

 coracoid process of the Mammalia, and the posterior clavicle of 

 Birds. It is for the most part longer, more pointed, and rib-shaped, 

 and, in the direction backward and inward, rests against the convex 

 part of the anterior clavicle, and converges frequently to that of the 

 other side ; it consists usually of two pieces, of which the superior 

 is broad and flat. 



To the above follows the second or middle division of the bones 

 of the thoracic fins, which is viewed as corresponding to the hu- 

 merus and antebrachial bones. This division consists of three, or 

 frequently only of two bones, perforated by openings, and often of 

 very considerable size. A row of smaller, flatter, and rarely more 

 elongated bones, follows the above division, and has, in conformity 

 with nature, been viewed as corresponding to carpal and metacarpal 

 bones. Four separate bones are mostly here present, but the num- 

 ber varies from two to five. A special division is in rare cases, 



