[MATTHEW] ACROTHYRA AND HYOLITHES 99 
either on the dorsal or ventral face.! In many cases the support of the 
muscles of Brachiopods is found on a thickened part of the shell, and 
this holds true for the angled part of the thickening tube of Hyolithes 
where the dorsal and ventral sides meet. 
Similarly on the ventral side of the tube, there is in quite a number 
of species of the early palæozoie forms, especially the Lower Cambrian, 
striæ or grooves that mark out the median area of the tube on this 
side, running from the apex to the orifice ; these in their position cor- 
respond to the scars of the cardinal muscles of the operculum. 
In the Ordovician and later Hyolithide (including one Upper Cam- 
brian species) there is developed a group of species, named Magnidorsati 
by Holm, in which the contracted (true) ventral side seldom shows any 
trace of the median area ; but nevertheless there are a few that by a 
ridge or ridges along the median line, show an equivalent to this 
median area.? 
If the conjecture that the vertical ridges on the inside of the tube 
of habitation in Hyolithes are related to the proximal ends of muscles 
which at the distal end were attached to the operculum, it would seem 
to imply that there were no oblique or transverse muscles in Hyolithes, 
but that the pull of the muscles (at least those which have been re- 
cognized) was directly into the tube, and there was no oblique or in- 
direct movement of the operculum, as there is in the dorsal valve of 
Lingulella, ete. 
Comparing the muscular system of Hyolithes with that of the 
Brachiopoda, we seem to find the following differences :— 



ACROTHYRA AND INARTICULATE 
HYOLITHES. 
BRACHIOPODS. 
WmbonalUSCIE EEE MERE MEN EE à Wanting. 
IBTANSMEMANS A ee eee me Cecile its Wanting. 
IRosteriorslateral Sec jester ce to eels a2 Present as the cardinal muscles. 
Miva ales ateral Si EEE NAS seers eaueuaes ee tenella vies Wanting or blended. 
Aniberior laterals RE EE APE iia edie EME ee Present and strong. 
Ambenionad du chorseeeener eaten RC CARTE PRE | Wanting or weak. 


1 Sverig. Kamb-Silur. Hyolith. och Conul., taf. 2, figs. 25, 26, 46 and 47; 
tafe, 6, fies. 112,13; 2428, etc. » Barr. Syst. Silur. vol. LEE; pl 12: figs 502 pli 13: 
figs. 39, 41 and 42; pl. 15, figs. 26, 28, 29, etc. 
“Holm op. cit., tafl. 2, figs. 25, 26; tafl. 5, figs. 2, 8 and 18. 
Seca Ves EL oO leis 
