0>f THE STRUCTURE AND CLASSIFICATION OP THE TbEMATASPIDAE. 29 



shell. The median eye was large, complex, and important functionally. It consisted of two 

 pairs of ocelli, one pair completely united in the median line and the other nearly so. The 

 lateral eyes are reduced in size and in functional importance. The nasal pit is unpaired and 

 lies behind the median eye. The mouth was small, circular, and situated near the centre of 

 a group of oral plates. No upper and lower jaws were present. 



The head of the Peltocephalata may be regarded as a modification of the cephalothorax 

 of an arthropodan ancestor, consisting of three principal groups of segments, namely : the 

 pre-oesophageal, including all the jiarts derived from the cephalic lobes ; the true thoracic 

 segments : and the highly modified vagus segments, formed by the forward migration of 

 from two to four abdominal segments and their complete union with the thorax. The brain, 

 like that of Vertebrates, probably consisted of three groups of neuromeres derived from 

 these three sources. 



The Peltocephalata have their nearest relatives among the known Invertebrates in the 

 Trilobites and the Merostomata, having retained the general shape of the body, structure 

 of the head, and mode of life characteristic of these Arthropods. 



Sufficient data are as yet unavailable for a permanent arrangement of the Peltocepha- 

 lata into orders and families, but some modifications of the old arrangement may be made 

 to advantage. The old subdivisions into Osteostraci and Heterostraci, proposed by Lanke- 

 ster, should be abandoned as they do not mark natural divisions. 



The discovery of heavily armored, oar-like appendages in Gyathaspis and Tremafaspis 

 and their probable presence in Pteraspis and Tolypaspis unite these genera more closely 

 with one another, and with the Asterolepidae than ever before. It is therefore inadvisable 

 to isolate the Pteraspidian section, merely on the absence of multipolar bone cells unless the 

 Pterichthydae are united with the remaining families under the heading Osteostraci. But 

 such an arrangement would not sufficiently emphasize the resemblance between the oar-like 

 appendages of Gyathaspis, Tremataspis, and Pterichthys, and the difference between these 

 appendages and those of Gephalaspis. 



Moreover the TH,eraspidae approach the Asterolepidae more closely than do the Trema- 

 taspidae^ in the division of the cephalic buckler into separate plates, and in its separation 

 into a true cephalic portion bearing the median eyes, and a thoracic one to which the oar- 

 like appendages are attached. On the other hand, Tolypaspis, which must be placed close 

 to Pteraspis and Gyathaspis on accout of the minute structure of the shield, shows no trace 

 of a subdivision of its dorsal shield into separate plates. 



It seems to me, therefore, that we must recognize four subdivisions of the Peltocepha- 

 lata of about equal value viz : The Pteraspidae, Tremataspidae, Asterolepidae and Gephalas- 

 pidae. The Gephalaspidae are separated from the other families on account of the very pe- 

 culiar shape of the head and the position and character of the appendages, although on the 

 other hand, as shown by the connecting form Thyestes, a close relationship between Gepha- 



