340 



Part III. — Eighteenth Annual Report 



organ is similar to that of the Halibut- and Pin ice-groups, that is the 

 laminae are arranged parallel to the longitudinal axis of the body. 



These are the main features of the olfactory organ in the various 

 groups of the Heterosomata. Sometimes, however, the epithelium is 

 much degenerated and the number of laminae reduced. In such cases, 

 e.g. ticophthalmus, a reason can usually be found in the habits of species 

 which have altered their sand-loving mode of life to some other where 

 the organ of smell is not of such great necessity. The main features 

 are sufficiently obvious, however, to render possible generalisations and 

 thence a classification based upon this character. 



With regard to the nasal sacs which have been described in a previous 

 paper (34), it may be said here that they seem to be special adaptations 

 and are of little importance for classificatory purposes. The nasal organ 

 of the eyed side only need be considered, because that of the other side 

 is distorted by its relationship to the dorsal fin, and the presence of one 

 of the nasal sacs is often doubtful. For the former organ, however, the 

 structures are quite clear. In the Turbot-, Plaice-, and Halibut-groups 

 there are generally two nasal sacs attached to this organ (Figs. 5,6; 

 In s., r.n.s.). In the Sole-group only one is usually present (Fig. 7, n.s.). 

 Exceptions occur in some of the groups, however, no sacs occurring in 

 several forms, e.g. Lepidorhombus, Pardachirus, Rhombosolea, so that no 

 very great importance can be attached to this character for the purposes 

 of classification. 



Pectoral Arch and Ventral Fins. 



The arrangement of the bones of the pectoral arch in the Hetero- 

 somata corresponds to the general Acanthopterygian type. The arch is 

 joined on to the skull by means of the post-temporal (p.t.) and the 

 supraclavicle (s.c). The post-temporal has two heads for articulation 

 with the otic region of the skull (a and a 2} Fig. 18, Plate XII.), 

 the upper being joined to the epiotic, the lower to the pterotic. Sometimes 

 it appears as if this bone were formed from two separate ossifications, 

 since it readily separates into two portions in the adult (Fig. 19, p.t. 

 and p.t.), but this occurs very rarely. The clavicle (cl.) begins 

 superiorly as a stout rounded bone, but gradually alters its form as 

 it passes the line of attachment with the scapula and coracoid and 

 becomes three-sided — two sides, external and internal, meeting in a 

 prominent anterior ridge, whilst the third, facing posteriorly, is concave. 

 This concavity is more marked in the Turbot -group than in the Plaice- 

 and Halibut-groups, because it continues to the ventral extremity of the 

 clavicle, whereas in the latter groups it gradually decreases until along 

 the line of junction of the one clavicle with the other the bone presents 

 only two surfaces, the one facing anteriorly, the other posteriorly. In 

 the Turbot- and also the Sole-group the pectoral arch is more bent or 

 curved anteriorly than in the Plaice and other groups (see Figs. 18, 

 19, and 20). This is noticeable externally in the forward curvature 

 of the ventral extremity of the pectoral arch in the Turbot-group, 

 and it is worthy of remark that Oitharus (Fig. 21) approaches more 

 nearly in this respect, as in many others, to the Plaice- and Halibut- 

 type than to the Turbot (cf. Fig. 23). This curvature of the clavicle is 

 connected with the position of the ventral fins. 



From the inner face of the clavicle superiorly arises the postclavicle. 

 This is not definitely fixed to the clavicle but lies loose in the 

 dermis between the muscles and the skin. It extends posteriorly within 

 the wall of the abdominal cavity as a whip-like bone (Fig. 19, p.c), and 

 probably has a similar function with regard to the abdominal wall that 



