BRACHIOPODA. 203 



persistent is the genus Lingula, which commences in the Cam- 

 brian Rocks, and has maintained its place up to the present 

 day, though it appears to be gradually dying out. 



According to Woodward : " The hingeless genera attainedN 

 their maximum in the Palaeozoic age, and only three now sur- 

 vive {Lingula, Distinct, Crania} the representatives of as many 

 distinct families. Of the genera with articulated valves those 

 provided with spiral arms appeared first, and attained their 

 maximum while the Terebratulidcz were still few in number. 

 The subdivision with calcareous spires disappeared with the 

 Liassic period, whereas the genus Rhynchonella still exists. 

 Lastly, the typical group, Terebratulidce, attained its maximum 

 in the Chalk period, and is scarcely yet on the decline." 



Of the families of the Brachiopoda, the Productidce and 

 Strophomenidce are exclusively Palaeozoic. The Spiriferidce 

 are mainly Palaeozoic, but extend into the Lias, where they 

 finally disappear. The Lingulidce commence in the Cambrian 

 period, and have survived to the present day. The Rhyncho- 

 nellidce, Craniadce, and Discinidce commence in the Silurian 

 period, and are represented by living forms in existing seas. 

 The Koninckinidce are exclusively Triassic. The Thecididcz ex- 

 tend from the Trias to the present day ; and the Terebratulida 

 appear to commence in the Devonian, and are well represented 

 by living forms. In the following are given the leading char- 

 acters and more important forms of the families of the Brachio- 

 poda : * 



FAM. I. TEREBRATULIDCE. Shell minutely punctate : ven- 

 tral valve with a prominent beak, perforated by a foramen for 

 the emission of a muscular ped- 

 uncle, whereby the animal is 

 fixed to some submarine object. 

 Foramen partially surrounded 

 by a deltidium of one or two 

 pieces. Arms entirely or par- 

 tially supported by calcified 

 processes, usually in the form 

 of a loop, and always fixed to ~ , 



' Fig. 143. Tj&ehratula. sacculus, Car- 



the dorsal Valve (fig. I43.) boniferous. The right-hand figure shows 



T,-. 4-1, ~ T< ~J * x.,/.~ "*. the interior of the dorsal valve with the 



In the genus Terebratula it- loop . (After Dawson .) 

 self, and in Terebratulina, the 

 loop supporting the arms is very short, the former commencing 



* Most of the details of this section are taken from the magnificent 

 Monograph of the Brachiopoda, published by the Palaeontographical 

 Society, and written by Thomas Davidson, Esq., F.R.S., the greatest 

 living authority upon the subject of the Brachiopods. 



