12 letters

acanthopanax coccothrinax glossanthrax pneumothorax ∙ blennothorax enterothorax gastrothorax hematothorax psychoreflex unimultiplex ultracomplex supercomplex hypercomplex ∙ heteroduplex ∙ Pogonomyrmex pseudocortex archeocortex mesoappendix Deinogalerix streptothrix perturbatrix vindemiatrix stereomatrix vaticinatrix regeneratrix perpetratrix dispensatrix conservatrix ∙ Platyhystrix chronomastix homeromastix scleromeninx hydrosalpinx metrosalpinx arthrosyrinx dacryosyrinx saccopharynx rhinopharynx retropharynx yogibogeybox hyperothodox semiorthodox antiorthodox philorthodox pseudocowpox portmanteaux ∙ fondaparinux Nautilocalyx aphanapteryx archeopteryx phyllopteryx Actinopteryx odontopteryx

13 letters

Saurophaganax Cephalothroax phalacrocorax Ultrabithorax Cervicothorax cephalothorax ∙ pyohemothorax schistothorax mechanoreflex immunocomplex ∙ concavoconvex convexoconvex hydroappendix pseudophoenix endoectothrix Vercingetorix significatrix versificatrix interrogatrix impropriatrix sermocinatrix exterminatrix impersonatrix sequestratrix improvisatrix ∙ primogenetrix primogenitrix ∙ interlocutrix ∙ histriomastix Hieracosphinx ∙ hematosalpinx electrolarynx ultraorthodox ∙ hyperorthodox Sharovipteryx archaeopteryx ∙ anomalopteryx

14 letters

InheritanceTax juxtallocortex englerophoenix Erysipelothrix Administratrix ∙ laryngopharynx counterparadox rickettsialpox pseudosmallpox chiropompholyx

15 letters

cholohemothorax hydrohemothorax pyopneumothorax pneumopyothorax cerebellocortex Archontophoenix lymphadenovarix physopyosalpinx cheiropompholyx Sinosauropteryx

16 letters

pneumohemothorax hemopneumothorax seropneumothorax pneumoserothorax uniprocessorunix coadministratrix christianomastix hydroparasalpinx archocystosyrinx

17 letters

chylopneumothorax Haemopneumothorax hydropneumothorax pneumohydrothorax pseudomonilethrix

Words with a ∙ are valid Scrabble words (SOWPODS word list)