On the strdctuke and CLASSiFicATiON OF THE Tremataspiuae. 17 



the shell is beut inward all rouud the slit to form a short vertical fuunel, whose outer end 

 opens into the bottom of the frontal pit and the inner end into the interior of the head. 



This depression has been regarded as the site of an olfactory organ (Schmidt) or para- 

 physis (Rohon). Its homology with a similarly located depression and slit-like opening in 

 Gephalaspis is not open to doubt. 



I regard, provisionally, the three anterior openings as the site of the parietal eye complex, 

 for the following reasons: (1) Because of the general similarity of these openings to the tri- 

 ocular median eye openings in Limulus and the Mcrostomata. (2) Because the assumption har- 

 monizes with the conclusion, based on anatomical and erabryological evidence, that the an- 

 cestral Vertebrates must have been provided with median eyes composed of at least two 

 pairs of ocelli that were of much greater functional importance than those of any recent 

 "Vertebrates. (3) The dumb-bell openings are not in the right place for the lateral eye or- 

 bits, as the latter are usually either on the same level as the median eye, or in front of it, 

 not behind it. (4) If the dumb-bell openings are the lateral eye orbits, there is no explana- 

 tion for the marginal openings. 



It is not surprising that the median eyes of Tremataspis are so much larger and more 

 important functionally than the lateral eyes, since this is the case in many Arachnids. 



The presence of au ocellus is not necessarily indicated either by a distinct opening, or 

 by a lens, since in TAmulus one pair of ocelli lies beneath a mere tubercle which in old spe- 

 cimens may be worn off completely. A similar condition seems to prevail in Pteraspis and 

 Tolypaspis. In the latter genus, the only evidence of a median eye is the presence of a 

 slight excavation on the inner surface of the dorsal shield (see p. 22). Between this con- 

 dition and that of Tremataspis where the orbits are apparently uncovered, we have forms 

 like Bothriolepis, with the orbits closed by hard plates, more suggestive of the cuticular 

 covering of an Arthropod eye than the ossified sclerotic of a Vertebrate. In Gephalaspis also, 

 the orbits were in some cases, asshownby the British Museum specimens, covered by promin- 

 ent dome-like continuations of the outer layer of the shell. 



These appearances indicate that the eyes were closed, not by mere ossification of the 

 sclerotic, but by a continuation of the outer layer of the shell to. form cuticular lenses or 

 corneas like those in an Arthropod. Ossifications of the sclerotic are as a rule characteristic 

 of much larger animals than the Ostracoderms, and ones in which there is a higher devel- 

 opment of the cndoskeletal tissues. Besides, I know of no case among true Vertebrates 

 where the ossification of the sclerotic extends over the whole front of the eyeball, as it 

 appears to do in Bothriolepis and Gephalaspis. 



In Limulus, there is a curious ingrowth, or infolding of the shell, over the proximal 

 end of the median eye tube where the latter joins the brain. In Apns the infolding is more 

 conspicuous and lies just in front of the median eye. The ingrowth closes a kind of anterior 

 neuropore, or the pore through which both the median eye tube and the fore-brain vesicle 

 opened to the exterior, before the final closure of the cerebral vesicle. The infolded 



San. <tH3.-MaT. Otj(. 3 



