GENERA OF THE SUBORDER ORTHOIDEA 



103 



vexity. In most species the fold and sulcus are usually 

 present only below the middle of the valves. The 

 fold is as a rule obscurely developed but in some old 

 individuals it may be marked. The beaks are always 

 closely appressed. In mature and old shells the beaks 

 are generally worn aw.ay due to resorption or abrasion 

 by the pedicle, so that a large round aperture occurs. 

 It was Davidson's belief that the beaks had been worn 

 away by friction in the opening and closing of the 

 valves. The condition is not unlike that seen com- 

 monly in Platystroph'm and other stocks with two obese 

 valves or with the rotundity of one greater than that of 

 the other. Continuous growth at the front margin 

 brings the beaks into closer apposition, squeezing the 

 pedicle, which must resorb or abrade the beaks, or have 

 its connection with the body cavity cut off by strangu- 

 lation. Beneath either beak is a short and narrow 

 interarea. The narrowness of the interareas in such 

 species as P. teretior, P. dejormatus, P. schmidtl, and 

 P. baueri gives them a subrostrate appearance. On 

 the cardinal slopes, outside the interareas, is a nearly 

 flat surface, elliptical in plan and bounded by a low 

 groove and ridge extending from the beak. Noetling 

 applied the name peudolunula to this area, since it had 

 been erroneously homologized with the lunule of cer- 

 tain pelecypods by early paleontologists. The pseudo- 

 lunula is morphologically of little significance but is 

 of use in species definition in Porambonkes. It marks 

 the progressive growth track of the end point of the 

 thickening of the lateral walls of the shell. 



Perhaps the most striking external feature of this 

 genus, and the one which gave the generic name, is 

 the peculiar surface ornamentation. This type of orna- 

 ment is, however, by no means unique in Porambonkes, 

 being present also in Linoporella and D'tctyonella. 

 The surface is covered by fine flat-topped radial ribs 

 which increase by bifurcation. The apparent pores 

 arranged in radial rows in the interspaces are produced 

 by dissepiments or cross-bars uniting the ribs. Each 

 dissepiment alternates with its neighbor in the adjacent 

 furrow, so that there is always a "pore" or depression 

 opposite a cross-bar. This combination produces a 

 fenestrated surface, resembling strongly the non-cellu- 

 liferous face of a few fenestellid bryozoa. This orna- 

 mentation is a feature of the external surface only. 

 The inner shell layer is fibrous and impunctate. In 

 many specimens this ornamentation does not survive 

 the rigors of life and it is completely or nearly com- 

 pletely worn off by solution or abrasion. This is 

 especially true of the more delicately ornamented 

 forms such as P. baueri, P. schmidti, and P. gigas. 



The features of greatest morphologic importance 

 and interest lie within the valves of Porambonkes and 

 have been interpreted in various ways according to 

 the superfamily in which one places these shells. In 

 individuals that have the beaks externally worn or 

 etched with acid, the internal dental plates and brach- 



iophore supports show as dark subparallcl or markedly 

 divergent lines. Internally in young and mature forms 

 these plates are sharp, discrete, and attached directly 

 to the floor of the valve. In some species, however, 

 P. schmidti for example, the dental plates converge 

 downward and forward quite definitely, but in their 

 attachment to the floor of the valve they remain dis- 

 crete. Late maturity and old age produce alterations 

 in the appearance of these plates, the importance of 

 which has been much discussed and considerably over- 

 emphasized. In late maturity Porambonkes spreads a 

 callus of adventitious shell over the posterior of the 

 shell, filling up the back portion of the delthyrial cavity 

 and obliterating the umbonal cavities. Great thicken- 

 ing takes place also at the front end of the dental 

 plates, the extra testaceous matter swelling them 

 inward, uniting them, and forming a short anterior 

 extension which simulates a median septum. In this 

 way the whole structure finally assumes the appear- 

 ance of a spondylium or sessile spondylium, and might 

 be taken for such if the ontogeny were not understood. 

 A similar development occurs in the dorsal valve. 

 These features of Porambonites seen in late maturity 

 and old age therefore can not be homologized with 

 the spondylium simplex of Clitambonites and not at 

 all with the spondylium duplex of Pentamerus. In 

 the former genus there has been an actual convergence 

 of the dental plates from the start (perhaps they were 

 never wholly discrete) and finally they unite with a 

 median septum. In Pentamerus the dental plates have 

 grown inwardly and have united, but there is always 

 a line of separation to show their discrete nature. 

 Porambonites therefore simulates the structure of 

 Clitambonites by its thick deposit of secondary shell, 

 hiding a primitive structure that is no spondylium at all. 

 The proper understanding of these features is thus 

 fundamental in establishing the correct taxonomic posi- 

 tion of the Porambonitidae. 



Davidson®* proposed the family Porambonitida? to 

 embrace these shells and concluded that their posi- 

 tion lay "between the Rhynchonellidcp and Stropho- 

 menidtp." In view of these changes Suess*' regarded 

 Porambonkes as a subgenus of Orthis. Quenstedt** 

 followed the views of Suess, placing the Porambonkes 

 species in his "Orthidas ventriplexae" along with Platy- 

 strophia lynx and Orfhisina anomala. Eichwald, 

 however, erected for the genus the family Poramboni- 

 tidx, which he placed between the Rhynchonellidaj and 

 the Strophomenid.-c following the genus Pentamerus. 

 Noetling's paper of 1883"* is by all means the best 



"Brit. Foss. Brach., vol. 1, Introd., 1851-1855, p. 99. 

 "Classification dcr Brachiopoden, 18 56, p. 112. 

 *' Petrefactenkundc Deutschlands, 1870, Bd. 2, Brach- 

 iopoden, p. 541. 



«' Lethia Rossica, Bd. I. Abth. 2, 1860, p. 793. 

 "'Zcits. deut. geol. Gesell., vol. 35, pp. 355-381. 



