[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 103 



I.4MELL.IBR4WCHIATA. 



MoDiOLopsis, Hall. 



MODIOLOPSIS THECOIDES (PI. V., figs. 7a to c.) 



Modiolopsis thecoides, Nat. Hist. Soc. N.B., Bull. xviiL, p. 191, pi. i., figs, la to c. 



A peculiar shell of which only the right valve is known, is referred 

 to the above genus on account of its general form, and the hinge. 



The valves are elongate, sub-cylindrical, broadly rounded in front» 

 and having the front end flattened as far as the umbo. The umbo is 

 distinct but not prominent. The valve gradually broadens and flattens 

 toward the posterior end, where it is broadly rounded. The greatest width 

 is about one-quarter or one fifth from the posterior end. There is a liga- 

 mental pit just opposite the umbo, and another pit a little further back 

 along the hinge line. The shell gaped somewhat at the end, the anterior 

 muscle-scar is indicated by a depression on the anterior flattened part of 

 the shell ; the posterior muscle-scar is apparently indicated by a sub-central 

 depression two-thirds from the umbo. 



Sculpture. — This consists of numerous arching striae of growth like 

 those of Hyolithes ; with about six stronger grooves marking a periodic 

 enlargement of the shell. 



Size. — Length, 9 mm. ; width at the umbo, 2 mm. ; at the posterior 

 end, 4 mm. ; depth of the valve, 1^ to IJ mm. 



The slow and regular enlargement of this shell from the umbo, the 

 arched strias ou the surface and the gaping of the end causes it to resemble 

 a Hyolithes. 



This shell in its small size and elongated form is unlike any Cambrian 

 or early Ordovician Modiolopsis. 



Since the publication of the reports of the Challenger Expedition we 

 have been accustomed to think differently of the Hyolithidœ than hereto- 

 fore. An author who has given the closest attention to the Pteropoda 

 and who has written the report on this class of animals for that publi- 

 cation, utterly repudiates any connection between the Hyolithidte and the 

 Pteropoda. This author, Dr. Paul Pelsineer, says : " I completely abstract 

 certain primary fossils, usually referred to the Pteropoda. I have a deep 

 conviction that these organisms do not really belong to the group in 

 question, and am firmly of the opinion that Pteropoda do not occur as 

 fossils until the end of the Lower Tertiary." ^ and farther on (page 92) 

 he adds : "In the Secondary Period [Mesozoic] there existed no Ptero- 



' Challenger Expedit., vol. 23, part ii., p. 7. 



