388 
ELEMENTS OE GEOLOGY. 
Fig. 413. 
Magnesian Limestone, Hnmbleton Hill, near Sunderland.* 
a. Fenestella retiformis, Schlot, sp. Syn. Gorgonia infundihuliformis, Goldf.; 
Retepora Jiustracea, Phillips, b. Part of "the same highly magnified. 
(Fig. 415) is an example. There are 25 gasteropods and only 
one cephtilopod^ Nautilus Freieslebeniy which is also found in 
the German Zechstein. 
Shells of the genera JProductus (Fig. 414) and Strophalosia 
(the latter of allied form with hinge teeth), which do not oc¬ 
cur in strata newer than the Permian, are abundant in the 
ordinary yellow magnesian limestone, as will be seen in the 
valuable memoirs of Messrs. King and Howse. They are ac¬ 
companied by certain species of Spiri/era (Fig. 416), Lingula 
Crednerii (Fig. 415), and other brachiopoda of the true pri¬ 
mary or pal 80 ozoic type. Some of this same tribe of shells, 
such as Camarophoria^ allied to Rhynchonella^ Spiriferina^ 
and two species of Lingula^ are specifically the same as fos¬ 
sils of the carboniferous rocks. Avicida^ Arca^ and Schizo- 
dus (Fig. 410), and other lamellibranchiate bivalves, are abun¬ 
dant, but spiral univalves are very rare. 
Fig. 414. 
Fig. 415. 
Fig. 416. 
Productus horridus, Sowerby. stone, and Car- Spirifera alata, Schloth. Syn. 
(P. calvus, Sow.) Sunderland boniferons Marl- Trigonotreta undulata, Sow., 
and Durham, in Magnesian slate, Durham; King’s Monogr. Magnesian 
Limestone; Zechstein and Zechstein, Thu- Limestone. 
Kupferschiefer, Germany. ringia. 
Beneath the limestone lies a formation termed the marl- 
slate, which consists of hard calcareous shales, marl-slate, and 
thin-bedded limestones. At East Thickley, in Durham, where 
. * King’s Monograph, PI. 2. 
