PISCES. 61 



Mouth a transverse fissure on the lower part of the head. Bones of 

 cranium not distinct by sutures. Copulation. 



Compare J. MUELLER und J. HENLE Systematische Beschreibung der 

 Plagiostomen. Mit 60 Steindrucktafeln. Berlin, 1841, folio. 



This order may in many respects be regarded as the most compo- 

 site and the most highly organised ID the class of fishes, whence 

 some recent writers place it at the top of the class and far from the 

 Cyclostomes. The conviction that it is an impossibility to preserve 

 the natural affinity, when animals are arranged in a single line, 

 has withheld us from making such a revolution in the previous and 

 more common arrangement of fishes. 



The cranium of these fishes presents no sutures. A single bone 

 attaches the lower jaw to the skull, taking the place of the jugal 

 bone, of the tympanicum, epitympanicum, and prceoperculum (see 

 above, pp. 20, 21). At the posterior margin of this bone cartilaginous 

 appendages are attached digitally, which correspond to the opercula 

 of the bony fishes. There are thin cartilaginous strips beneath the 

 skin which support the margins of the external branchial apertures, 

 and replace the more composite apparatus of the Petromyzonines. 

 Hence it is obvious that the apparatus in Petromyzon is not homolo- 

 gous with branchial arches. 



Above, on the head, with few exceptions, are two apertures 

 behind the eyes, in front of the quadrate or suspensory bone of the 

 lower jaws ; they conduct to the mouth and transmit the water that 

 has been distributed to the gills (foramina temporalia, events, 

 Spritz-locher). A real copulation occurs in these fishes; compare 

 above, pp. 39, 40. All have a spiral valve in the intestinal canal. 



Family IV. Batides. Body depressed. Branchial apertures 

 on the neck below, five on each side. Eyelids connate with the 

 eyes or none. Cartilaginous belt sustaining the pectoral fins adher- 

 ing above to the vertebral column. 



This family consists principally of the genus Raja L., to which, 

 as MUELLER has remarked, Squalus Pristis L. might be added, 

 which forms indeed the transition to the following family, but 

 still agrees with the rays in essential characters : the branchial 

 apertures are situated on the ventral surface, &c. 



In this family the dorsal fins are commonly far backwards. In 

 some species of rays individuals occur with a membrane on the 

 middle of the disc of the body, as in Raja clavata; such varieties 



