650 



MOLLUSCOIDEA. 



muscles only, the mantle-lobes are completely free, and the intestine 

 terminates in a distinct anus. This order includes the Palaeozoic 

 families of the Obolidaz and TrimerellidcB, and the three families of 

 the Lingulidtz, Discinidce, and Craniadce, which commence in the 

 Lower Palaeozoic rocks and are all represented by living species at 

 the present day. 



Family i. Lingulid^. — In this family the animal is fixed by a 

 muscular peduncle which passes out between the beaks of the valves 

 (fig. 486, b). The shell is almost equivalve, oblong or ovate in 

 form, and composed of alternating corneous and calcareous layers, 

 the latter being phosphatic and perforated by minute tubuli. The 

 arms are fleshy, spirally rolled, and not supported by calcified pro- 

 cesses. The family includes the genus Lingula along with some 



allied (sub-generic ?) forms, 

 and ranges from the Lower 

 Cambrian to the present day. 

 In the genus Lingula (fig. 

 494) the shell is oblong, 

 compressed, the dorsal valve 

 little shorter than the ven- 

 tral. The shell is ova], 

 rounded, or satchel-shaped, 

 tapering more or less to- 

 wards the beaks. The sur- 

 face is concentrically striated 

 with lines of growth. The 

 genus commences to be rep- 

 resented in the Cambrian rocks, and has continued without interrup- 

 tion, and with no perceptible change, to the present day. 



Fig. 494. — Lingula Eva. 

 ventral valves. 



Ordovician. Dorsal and 

 (After Billings.) 



The genus, or sub-genus, Lingulella (fig. 495, d) differs from Lingula 

 proper in the fact that there is a distinct groove in the beak of the ventral 

 valve for the passage of the peduncle. This type comprises the oldest 

 of British Brachiopods, L. ferrngiiiea being a small form which occurs 

 low down in the Lower Cambrian. Another species — viz., L. Davisii — 

 is extremely abundant in the so-called "Lingula Flags" (Upper Cam- 

 brian). The sub-genus Lingulepis is very similar to Lingula proper, 

 but the ventral valve is furnished with a feeble median septum. The 

 type is the L, fiinnceforntis of the Cambrian rocks of North America. 



Family 2. Obolid^:. — This family comprises forms closely allied 

 to the preceding, but the beaks of the valves are thickened at their 

 margin, and that of the ventral valve is provided with a groove for 

 the passage of the peduncle. In structure, the shell is calcareo- 

 corneous. The family is principally Cambrian, Ordovician, and 

 Silurian, the genus Schmidtia surviving into the Devonian period, 



