[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 59 



European OgygifB, nor so close to the glabella. Tn this the American 

 form shows a less advanced stage of development, and so perhaps belongs 

 to an older fauna. The oldest European Ogygia is of the Tremadoc 

 group,* = Dicellocephalus Fauna. 



The thorax is wider near the pygidium than at the head-shield ; 

 and the geniculation of the pleurœ, where the pleurœ bend backward, 

 recedes further and further from the axis in going backward. 



Sculpture. — The surface of the test is very finely granulated. 



There appears to be a wide and a narrow form of this species, in- 

 dicated by the comparative width of the rachis and lateral expansions of 

 the body ; in some examples the side lobes of pygidium are nearly twice 

 as wide as the rachis, in othei'S they are only one and a half times as 

 wide. 



Development of the young. — Koeminger examined tests of this species 

 from a length of 16 mm. upward. In Mr. Walker's collection some of 11 

 mm. and one of 9 mm. were observed ; the younger ones show some 

 differences from the adult. In the 11 mm. tests (one tenth of the length of 

 of the adult) there are tubercles on the thoracic rings for the attach- 

 ment of spines ; in one example these spines are attached to back of the 

 rings ; that of the occipital ring crosses two joints of the thorax, and that 

 on the last joint of the thorax crosses three rings of the pygidium. In 

 the adult all these spines have disappeared except that the fifth joint is 

 sometimes seen to have a short spine. In the young shields the genicula- 

 tion of the pleurte, and the relief of the rachis are more pronounced than 

 in the adult. 



In the 11 mm. test there are 8 joints in the thorax and 10 in the 

 pygidium, and 8 costœ on the side lobes of the latter. At this age then 

 the species had the full number of joints in the thorax, but lacked two of 

 the full number in the pygidium. Tests of 22 mm. length had an 

 additional joint in the pygidium, and in tests 35 mm. long, the full 

 number was attained. 



A difference from the adult is observable in the length of the eye-lobe 

 of the young : in these the proportion between the length of the anterior 

 extension of the suture of the eye-lobe, and of the posterior extension are 

 respectively 1, 1, 2 ; in the adult it is 1^, 1, 2^ or 3. This contraction of 

 the eye-lobe is in accordance with what has been observed in other genera, 

 e. (J. Paradoxides. 



1 Highest Cambrian of English Geologists. Lowest Ordovician of the Swedes 

 and Germans. 



