130 NEW YORK STATE MUSEUM 



eye. . . The parietal eye usually persists through life, and it 

 may be the largest and most important one functionally." 



Our observations on the median tubercles in trilobites lead to 

 the conclusion that they represent median eyes in very different 

 stages of development, from mere transparent spots of the test in 

 most genera to fully developed lenses in others. The eye tubercles 

 in Isotelus gigas which gradually disappear with advancing 

 ontogenetic development, are hollow and possess obviously no more 

 than a transparent spot on the top of the small tubercle, underlaid 

 by a pigment spot, and may have served only for the recognition 

 of light intensities. This primitive type of parietal eye is alike to 

 the well-recognized eye tubercles of many fossil crustaceans, 

 notably of such ostracods as Leperditia or phyllopods as Ceratio- 

 caris. The eye tubercles of Leperditia p. e. are minute tubercles 

 (see Ulrich, no. 28, p. 635), hardly noticeable, always showing, 

 however, a pit on the inside. 1 While this is probably the prevailing 

 type of the eye tubercles in the Asaphidae, it is not the only one. 

 The excellent preservation of the Paleozoic fossils in the Russian 

 Baltic provinces, shown in the brachiopods of the Cambrian and the 

 justly famous eurypterids of the Silurian, is also manifest in the 

 uncarbonized chitinous exoskeleton of the Ordovician trilobites. 

 We had unfortunately only few specimens at our disposal, namely, 

 examples of Asaphus expansus (Linne) and A . rani- 

 c e p s Dalman. In one of the former the top of the tubercle is 

 broken off, showing the latter to be hollow ; in another the tubercle 

 possessed a transverse flat top, declining forward, which showed 

 two darker spots, arranged transversely. On sectioning this speci- 

 men it was found that in these spots the test was slightly thinner 

 and that the opaque matrix there reached closer to the surface 

 causing the darker spots (see pi. 35, fig. 9). In a third, as also in a 

 specimen of A. raniceps, the interior cast of the tubercle 

 exhibits a distinctly flat top. In the other specimens of A . 

 expansus it is depressed with a well-defined ring (see postea, 



P- 133)- 



Schmidt's (no. 27) beautiful photographs of Asaphidae in his 

 " Revision der ostbaltischen silurischen Trilobiten," afford instances 

 of interior casts of eye tubercles where a well-defined pit or pore 



1 Deecke (no. 6, p. 116) states that one can make thin sections through 

 the eye tubercles of the Leperditias of Gotland, adding (translated) : " There 

 the otherwise dark shell is light-colored, but passes in a thinner condition 

 over the tubercle, so that, if eyes were under them, the animals looked through 

 the shell, which happens quite often in crustaceans." 



